Method and apparatus for computer aided surgery

ABSTRACT

A number of improvements are provided relating to computer aided surgery. The improvement relates to both the methods used during computer aided surgery and the devices used during such procedures. Some of the improvement relate to controlling the selection of which data to display during a procedure and/or how the data is displayed to aid the surgeon. Other improvements relate to the structure of the tools used during a procedure and how the tools can be controlled automatically to improve the efficiency of the procedure. Still other improvements relate to methods of providing feedback during a procedure to improve either the efficiency or quality, or both, for a procedure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/831,691, filed Aug. 20, 2015, titled “Method and Apparatus forComputer Aided Surgery”, which is a continuation of U.S. patentapplication Ser. No. 11/927,429, filed Oct. 29, 2007, titled “Method andApparatus for Computer Aided Surgery”, Publication No.US-2008-0077158-A1, which is a continuation-in-part of U.S. patentapplication Ser. No. 11/764,505, filed Jun. 18, 2007, titled “Method andApparatus for Computer Aided Surgery”, now U.S. Pat. No. 8,560,047,which application claims priority from U.S. Provisional Application No.60/814,370, filed Jun. 16, 2006, titled “Method and Apparatus forComputer Aided Orthopaedic Surgery”, and U.S. Provisional ApplicationNo. 60/827,877, filed Oct. 2, 2006, titled “Method and Apparatus forComputer Aided Surgery”. Each of the foregoing applications is herebyincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of computer assisted surgery.Specifically, the present invention relates to various aspects of asurgical suite in which a computer provides guidance or assistanceduring a surgical procedure.

BACKGROUND

Many surgical procedures are complex procedures requiring numerousalignment jigs and intricate soft tissue procedures. Preparing andplacing the alignment jigs and other preparation is often a significantpart of the procedure. For instance, when performing a total kneereplacement procedure (“TKR”), the prosthesis must be accuratelyimplanted to ensure that the joint surfaces are properly aligned. If thealignment is inaccurate, the misalignment will lead to failure of thejoint, requiring the complex task of replacing one or more portions ofthe knee prothesis.

To ensure that the prosthesis is accurately implanted, during a TKRprocedure, the surgeon uses a variety of jigs to guide the cutting ofthe femur and the tibia. The jigs are complex devices that requiresignificant time to install on the patient during the surgicalprocedure.

The advent of computer assisted surgery provides the promise ofsimplifying many of the complexities of surgical procedures. In someinstances, the computer may be used to guide the surgeon during theprocess. Although computer assisted surgery holds promise, there arenumerous aspects to be addressed to make a system commercially viable.For instance, in addition to improving the efficiency of the procedures,the quality of the resulting procedures should be addressed.Accordingly, there continues to exist numerous aspects of computerassisted surgery that require improvement to improve the efficiencyand/or quality of the procedure. The end result will encourage medicalprofessionals to migrate toward computer assisted surgical systems.

SUMMARY OF THE INVENTION

In light of the foregoing, a computer assisted surgical suite having anumber of improvements is provided. For instance, a surgical suitehaving a computer and a surgical tool that communicates with thecomputer may be provided. The system also includes a tracking elementfor tracking the position of the surgical tool. In one aspect, thesystem allows the surgeon to perform a surgical procedure on a virtualmodel of the patient using the surgical tool. As the surgeon performsthe procedure on the virtual model, the computer stores the informationregarding the sequence of the steps performed and the position of thesurgical tool during the procedure. Once the surgeon is satisfied withthe results on the virtual model, the stored information can be usedduring the procedure to assist or guide the surgeon.

According to a further aspect, the computer controls operation of thesurgical tool in response to information detected regarding the surgicaltool. For instance, the system may track the position of the surgicaltool relative to the patient. Based on the data regarding the positionof the surgical tool, the computer may send signals to the surgical toolto control the operation of the surgical tool, such as reducing thespeed on the tool or turning the tool on or off.

According to another aspect, the system provides a communication linkbetween the surgical tool and the computer system that allows thesurgical tool to control operation of the computer system and thecomputer system to control operation of the surgical tool.

Another aspect of the system is directed toward the use of the surgicaltool in a free hand procedure to reduce or eliminate the use of jigsduring a procedure. In such a procedure, the computer tracks theposition of the surgical tool relative to the patient and displays theresults on a screen to guide the surgeon in the procedure. In aresection procedure, the system may be configured to identify thepatient tissue with different colors to identify the proximity of thetissue to the resection boundaries. For instance, tissue that is not tobe resected may be illustrated in a red color, so that the surgeon caneasily see that the tissue is not to be resected. Tissue that is to beresected may be illustrated in a green color. Further, tissue at theboundary of the portion to be resected may be illustrated in yellow, sothat the surgeon can easily see that the cuts are getting close to theboundary.

Yet another aspect of the system is directed toward improving thedisplay of information during a surgical procedure. Specifically,depending on which portion of a procedure is being performed, thesurgeon may desire to change the view of the information beingdisplayed. It can be cumbersome to change the view in the middle of aprocedure to a different view. Accordingly, the system can be used toautomatically switch to a particular view based on the position of thesurgical tool. Additionally, the surgeon may program this informationbefore a procedure, or the system can learn to recognize that aparticular surgeon desires a particular view based on inputs from thesurgeon during various procedures.

According to a further aspect, the system provides a method forassessing and improving the quality of a bone cut. For instance, thesystem measures various parameters relating to the quality of a bonecut, such as surface roughness, accuracy of each cut. If the parameterfall within pre-defined limits, the system indicates to the surgeon thatthe resection was successful, so that the prosthesis can be implanted.If one or more parameter falls outside the pre-defined limits, thesystem may calculate the step or steps necessary to correct the bonecuts so that the surgeon can perform the necessary correction.

Another aspect of the invention is directed improving the monitoring ofthe surgical tool. For instance, in certain aspects of computer assistedsurgery, the position of certain surgical tools may be quite importantin assessing the steps necessary during the procedure. However, duringthe procedure, operation of the surgical tool may cause the tool todeflect. The deflection may result in the system misidentifying theactual position of the surgical tool. Accordingly, the present systemmay include one or more sensors for detecting deflection of a portion ofthe surgical tool and an element for modifying the tracking element inresponse to the detected deflection.

A still further aspect of the present invention is directed to a markerthat is used for marking tissue to be resected. The marker includes anactuator that responds to signals from the computer system. A trackingelement provides data to the computer regarding the position of themarker. Based on the position of the marker, the computer controls themarker between an extended position and a retracted position.Specifically, if the computer detects that the marker is on a portion ofthe patient that is to be marked, then the computer controls the markerto extend the marker to the extended position so that a tip of themarker is exposed to mark the patient. Alternatively, if the marker ison a portion of the patient that is not to be marked, the computercontrols the marker to retract the tip of the marker so that the markercannot mark the patient.

DESCRIPTION OF THE DRAWINGS

The foregoing summary and the following detailed description of thepreferred embodiments of the present invention will be best understoodwhen read in conjunction with the appended drawings, in which:

FIG. 1 is a diagrammatic view of a computer assisted surgical suite.

FIG. 2 is a diagrammatic view of a surgical tool of the surgical suiteof FIG. 1 .

FIG. 3 is an alternative diagrammatic view of a computer assistedsurgical suite.

FIG. 4 is a fragmentary view of a surgical tool of the surgical suite ofFIG. 1 .

FIG. 5 is an alternative embodiment of the surgical tool illustrated inFIG. 4 .

FIG. 6 is plot illustrating data regarding the surface roughness andsurface waviness.

FIG. 7 illustrates a separation of the surface waviness and surfaceroughness of a surface profile.

FIG. 8 is a table illustrating the various potential error in fitting animplant.

FIG. 9 is a measuring block for assessing the fit of an implant.

FIG. 10 illustrates the femur cuts for a total knee replacementprocedure.

FIG. 11 is a diagram illustrating the error angle for bone cuts in atotal knee replacement procedure.

FIG. 12 is a diagram illustrating the steps of a method for programminga surgical robot.

FIG. 13 is a diagrammatic illustration of a navigable marking pen.

FIG. 14 is a registration block for registering tools of a surgicalinstrument.

FIG. 15 is a registration pointer operable in connection with thesurgical suite illustrated in FIG. 1 or FIG. 3 .

FIG. 16 is an alternative embodiment of a surgical tool operable inconnection with the surgical suite of FIG. 1 or FIG. 3 .

FIG. 17 is a block diagram of the wireless features of the surgicalsuite illustrated in FIG. 3 .

FIG. 18 is a top view of an alternative cutting blade operable inconnection with a surgical saw.

FIG. 19 is a bottom view of the cutting blade illustrated in FIG. 18 .

FIG. 20 is a perspective view of a surgical instrument having a housingwith embedded markers and an onboard screen in line with the cuttinginstrument.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures, wherein like elements are numbered alikethroughout, a surgical suite for computer assisted surgery is designatedgenerally 50. The suite 50 includes a first computer 70 forpre-operative use. For example, pre-operative analysis of the patientand selection of various elements may be performed on the firstcomputer. The suite may also include a second computer 80, referred toas the OR computer, which is used during a procedure to assist thesurgeon and/or control one or more surgical instruments. In addition thesuite may include a computer (standalone or collaborating with 80)mounted on the surgical instrument. First computer 70 is provided in thepresent instance, but may be omitted in some configurations because thefunctions of computer 70 are also implemented on OR computer 80, whichcan be a standalone. Moreover the whole ‘pre-surgical planning’ mayhappen instantaneously inside the OR. Nevertheless, if desired forparticular applications, first computer 70 may be used. Furthermore, themicro-processing system of the system 50 can reside in the cuttinginstrument. In such a configuration, the computations and user interfacecan be performed within a computer on the surgical tool. Such systemperforms error analysis of location of the cutting instrument relativeto the ideal cut to be performed, and displays corrective actions andother information on a screen mounted to the instrument.

The suite 50 may include a tracking/navigation system that allowstracking in real time of the position in space of several elements,including: (a) the patient's structures, such as the bone or othertissue; (b) the navigable surgical tools, such as the bone saw 100,which is controlled by the surgeon based on information from the ORcomputer 80 or (c) surgeon/assistants system specific tools, such as apointer, registration tools, or other objects. The OR computer 80 mayalso perform some control on the cutting instrument trough theimplemented of the present configuration of the system. Based on thelocation of the tool, the system 80 is able to vary the speed of thesurgical tool 100 as well as turn the tool off to prevent potentialdamage. Additionally, the suite 50 may also include a surgical robot 200that is controlled by the OR computer 80. The features of the navigabletool 100 and the surgical robot 200 may vary. The details of severaldesirable features are described in greater detail below. The variousfeatures can be selected as desired for a particular practice orsituation. In the following description, the only surgical instrumentshown in figures is the navigated saw 100. Nonetheless, many othersinstruments can be controlled and/or navigated as explained above, suchas a drill, burr, scalpel, stylus, or other instrument. Therefore in thefollowing discussion, the system is not limited to the particular tooldescribed, but has application to a wide variety of instruments.

As discussed further below, one exemplary use of the surgical suiteincorporates the use of a virtual model of the portion of the patientupon which a procedure is to be performed. Specifically, prior to aprocedure, a three dimensional model of the relevant portion of thepatient is produced using CT scans, MRI scans or other techniques. Priorto surgery, the surgeon may view and manipulate the patient model toevaluate the strategy for proceeding with the actual procedure.

One potential methodology uses the patient model as a navigation deviceduring a procedure. For instance, prior to a procedure, the surgeon mayanalyze the virtual model of a portion of the patient and map out thetissue to be resected during a procedure. The model is then used toguide the surgeon during the actual procedure. Specifically, during theprocedure a tracking mechanism monitors the progress of the procedureand the results are displayed in real time on the OR computer 80 so thatthe surgeon can see the progress relative to the patient model.

Referring to FIGS. 1-2 , to provide navigation assistance during aprocedure, the system 50 includes a position detection device 120 thatmonitors the position of the surgical tool 100. The surgical tool 100includes one or more position markers 105 that identify pre-definedpoints of reference on the tool. In the present instance the surgicaltool includes several markers 105 which, together with some pre-definedpoints of reference on the tool, identify the tool and its location.

Although a variety of position tracking systems can be used, oneexemplary system is the NDI Polaris optical measurement system producedby Northern Digital Inc. The system uses a position sensor and bothactive and passive markers. The active markers may be wired sensors thatare electrically connected to the system. The active markers emitinfrared light that is received by the position sensor. The passivemarkers are wireless markers that need not be electrically connected tothe system. The passive markers reflect infrared light back to theposition sensor. Typically, when using passive markers, the positionsensor floods the field of view with infrared light that is thenreflected back to the position sensor from the passive markers. Theposition sensor includes an infrared receiver and it receives lightemitted light from the active markers and reflected light from thepassive markers. The position system triangulates the three dimensionalposition of the tool based on the position of the markers. In thepresent instance, the position detection device 120 is also operable todetect the orientation of the tool relative three orthogonal axes. Inthis way, the position detection device 120 determines the location andorientation of the tool 100.

The position detection device 120 is linked with the OR computer 80 sothat the data regarding the position of the surgical tool 100, thepatient's anatomy, and other system specific tools, is communicated tothe OR computer. The computer uses this information to track theprogress of a procedure.

To track the position of the surgical tool 100 relative to the patient,a position marker is attached to the portion of the patient on which theprocedure is to be performed. The position marker attached to thepatient may be similar to the position marker 105 attached to thesurgical tool 100, as shown in FIG. 4 . The position marker on thepatient is correlated to a corresponding point on the virtual model ofthe patient. In this way, the registration point positions the toolrelative to the patient and the patient relative to the virtual model.

A series of points are used to register or correlate the position of thepatient's anatomy with the virtual model of the patient. To gather thisinformation, a navigated pointer is used to acquire points at ananatomical landmark or a set of points on a surface within the patient'sanatomy. A process referred to morphing may be used to register thepatient to the virtual model of the patient. During such a process, thesurgeon digitizes parts of the patient and some strategic anatomicallandmarks. The computer 80 analyzes the data and identifies commonanatomical features to thereby identify the location of points on thepatient that correspond to particular points on the virtual model.

Accordingly, as set forth above, the position detector monitors theposition of several items in real time, including: the position of thesurgical tool 100, the position of the patient and the position of itemsused during a procedure, such as a pen or marker as described furtherbelow. Accordingly, the computer combines the data regarding theposition of the surgical tool 100, the data regarding the position ofthe patient, and the data regarding the model of the patient. Thiscombination is used to provide a real time model of the position of thetool relative to the patient, which can be viewed by the surgeon on themonitor. Further still, as previously described, prior to a procedure,the surgeon may analyze the patient model and identify the tissue thatis to be resected. This information can then be used during theprocedure to guide the surgeon.

During the procedure, the monitor displays a model of the surgical toolrelative to the patient model, which reflects the real time position ofthe tools, such as the surgical tool 100, relative to the patient. Thesurgeon can align the position of the tool 100 by viewing the positionof the image of the tool relative to the patient model on screen. Oncethe monitor shows the virtual tool to be aligned with the portion of thepatient model identified for resection, the surgical tool is properlyaligned on the patient. In this way, the doctor can align the toolwithout the need for complex jigs or fixtures. Further, as the tool 100intersects the patient, the data regarding the position of the tool andthe patient model is correlated to show the result of the toolintersecting the patient. In this way, the computer can analyze anddisplay the progress of a procedure in real time. As the tool 100 cutspatient tissue, the monitor displays the tissue being removed from thepatient model. Therefore, in addition to guiding the position of thetool, the OR computer can be used to guide the surgeon as to what tissueshould be resected during a procedure.

In addition to including a surgical tool controlled by the surgeon, thesuite 50 may include a surgical robot 200. The surgical robot can beprogrammed to perform one or more operations during a medical procedure.The surgical robot 200 is controlled by the OR computer, which isprogrammed with the instruction set for the procedure. As with thenavigation system described above, when using the robot, the positiondetection device 120 monitors the position of the surgical robot, andprior to the procedure the location of the patient is identified so thatthe computer has data regarding the position of the surgical robotrelative to the position of the patient.

Controlling the Selection of View

As previously discussed, information regarding the position of thesurgical instrument can be combined with a model of the patient to guidethe surgeon during a procedure. During a particular step in a procedure,the surgeon may desire to see a particular view or perspective with acertain combination of orientations of the patient model and theinstrument, as well as two dimensional projections. For example whenmaking planar bone cuts using an oscillating bone saw, the system mayshow simplified two dimensional views along with a three dimensionalview. The simplified two-dimensional diagrams may be similar to thosedisplayed on aircraft cockpits or flight simulators, which help alignthe cut and correct roll and pitch of the saw relative to the plane tobe cut. Such diagrams dynamically update in real time and can be shownon the main display of the system and/or a secondary screen, includingone mounted on the cutting instrument itself. Although a surgeon maydesire to have control over the view, the surgeon normally does not wantto be encumbered by manipulating system controls during a procedure.Accordingly, the system is configured to quickly and efficientlyautomatically change the relevant views and perspectives of the surgicalscene. Additionally, the system allows the surgeon to manually selectany view/perspective the surgeon may desire.

In the present instance, the system is operable in three modes:

a) manual view selection;

b) pre-set view selection; and

c) automatic view selection.

Each of these is described further below.

Manual View Selection

In the manual view selection mode, the surgeon selects from amongnumerous parameters to define the exact view that the surgeon desires.The parameters include: the orientation (therefore perspective) of the3D surgical scene as rendered on the computer screen in the form of an“image”, the orientation of the patient model, the surgical instrumentto be illustrated, the zoom ratio of the image, the panning position ofthe image, the transparency of the patient anatomy models, thetransparency of the surgical instrument, and the coloring of variousitems on the screen. These parameters are only examples of the types ofparameters that that the surgeon can control to define a view. Thesystem may include many other parameters that the surgeon can control.

In the manual view selection, the surgeon selects and adjusts one ormore of the parameters to define the view. For example, the system mayinclude controls to rotate the image about one or more axes. The surgeonmay rotate the image through varying degrees about one or more axes todisplay a particular orientation to see a particular feature of thepatient. In this way, the surgeon can precisely define the orientationof the image. In addition to controlling the image orientation, thesurgeon can control any other parameter to define a view.

The system may include a variety of input devices in addition to orinstead of a typical computer mouse and/or keyboard to allow the surgeonto enter the information to define the parameters for a particular view.For instance, the view screen may be a touch screen and the surgeon mayenter the information through one or more menus. Alternatively, thesystem may include a voice recognition element so that the surgeon mayenter the parameters by voice. Further still, the surgical instrumentitself may be configured with controls that can be used to enterinformation regarding one or more of the parameters. In short, thesystem may include one or more of a variety of interfaces to allow thesurgeon to input information to control the parameters that define aview.

Pre-Set View Selection

Although the manual view selection mode provides precise control of thevarious parameters that define the view for an image, the manual modecan be cumbersome if the surgeon must manually define the parameters foreach view during a procedure. To make the view selection easier for thesurgeon, the system may also include a pre-set view selection mode.

In the pre-set view selection, the surgeon selects a view from a list ofpre-set views. A pre-set view is one in which the parameters that definea view have been previously selected. The pre-set views are eitherprogrammed as defaults for the system, or are added as custom views bythe user. For instance, a pre-set view may show the patient model in aparticular orientation or perspective at a particular zoom and pan, witha particular bone transparency and coloring. Since the parameters arepre-set, the surgeon can simply select a pre-set view to see the imagemodel. In this way, the surgeon can easily select from a number of viewsto quickly and easily view the patient model under a variety ofconditions.

By way of a simple example, in the case of a procedure to implant a kneeprosthetic, the pre-set views may include a set of views of the femur,such as an anterior view, a posterior view, a medial view, a lateralview, and a distal view. The surgeon can quickly select from among theseorientations simply by selecting one of the pre-set views. In thisexample the views are defined by the orientation of the patient model,however, each pre-set view includes a number of pre-set parameters (suchas a suitable pan, zoom, relative transparency of the different objects,etc), the combination of which is designed to provide an optimizedgraphical environment for a particular bone preparation/cutting partwith the surgical instrument, bone cut assessments, measurements, etc.

As described above, the system may include a variety of differentelements to allow the surgeon to select those parameters for the view.For instance, the system can use computer voice recognition or a touchscreen or controls mounted directly on the surgical instrument to inputdata regarding the view. Any of these or other input mechanisms can beused to allow the surgeon to select a pre-set view.

After the surgeon has selected a pre-set view, the surgeon may thenmanipulate one or more parameters to alter the view. Referring again tothe list of pre-set views for a knee procedure, the surgeon may selectan anterior view that illustrates, for example, the front of the femur.The pre-set view may be defined such that the patient model isillustrated at 100% magnification. After selecting the pre-set view thesurgeon may alter the view to increase the zoom to 200% magnification.The surgeon can make this change using any of a variety of controls thatthe system may include, as discussed above.

In the present instance, the system also includes the ability to add aview to the list of pre-set views. For example, as described above, thesurgeon can use the manual view selection mode to define a view and thensave the view as a pre-defined view. Similarly, the surgeon may select apre-set view and alter one or more parameters to create a new view. Thesurgeon may then save the new view as an additional view.

For instance, as described above, the surgeon may select a pre-set viewentitled anterior view to show the front of the femur. After increasingthe magnification of the pre-set view to 200%, the surgeon may choose tosave the new view as “enlarged anterior view”. Therefore, the surgeoncan readily bring up the anterior view with the magnification at 200%simply by selecting the view entitled “enlarged anterior view”. Althoughthis example demonstrates how a pre-defined view can be created toprovide a view with a different magnification, it should be appreciatedthat a new view may be defined having whatever combination of parametersthe surgeon desires.

Automatic View Selection

Although the pre-set view mode makes it easier for a surgeon to select adesired view during a procedure, the surgeon must still take the step ofselecting the view that the surgeon wants displayed. Since a surgeonnormally will frequently switch among various views during differentsteps of a procedure, it may be desirable to provide an easier way ofswitching the view during a procedure. Therefore, the system may alsoinclude a mode in which the view is automatically selected by thecomputer depending on what part of the bone preparation process thesurgeon intends to perform.

In the automatic view selection mode the system automatically selectswhich view to show based on the current location of the surgical (ormeasuring) instrument relative to the bone. Specifically, as discussedpreviously, the system includes a mechanism for tracking the location ofthe surgical instrument relative to the patient. In response to the dataregarding the position of the surgical instrument relative to thepatient, its previous location, its approach to a new location and itsmotion, the system determines what the surgeon is attempting to do. Thesystem then determines the appropriate/optimal view, and shows the viewon the display.

Referring again to the example of the knee procedure discussed above, ifthe surgeon positions the surgical instrument so that the cutting bladeis close to the anterior portion of the femur and the plane of the bladeof a cutting saw is approaching parallel to one of the desired planarcuts (such as the anterior cut), the system will detect the proximity ofthe instrument relative to the patient, and the orientation of the bladerelative to the patient. Based on the proximity, orientation, andapproach of the instrument to the anterior portion of the femur, thesystem will select the optimum view for this cutting process and showthat view on the view screen. In this case that optimum could be (butnot limited to) a sagittal view (lateral or medial) of the femur withpan, zoom and other transparency settings of bone and blade to focus onthe anterior cut being performed. If the surgeon moves the surgicalinstrument so that the cutting blade is approaching closely and in theright orientation to perform the posterior cut of the femur, the systemwill sense the change in position and change the view to show the viewappropriate for that specific cut.

Although the system has been described as automatically selecting a viewin response to the position of the surgical instrument, other data canbe incorporated into the determination of the view that is automaticallyselected. The additional data may be information input into the systemby the surgeon or otherwise. For example, referring again to the kneeexample, the surgeon may prefer to see the “enlarged anterior view” whenthe surgical instrument is adjacent the anterior portion of the femurrather than the default “anterior view”. Therefore, the surgeon mayinput information directing the system to use the “enlarged anteriorview” instead of the default “anterior view”.

Accordingly, during the procedure, when the surgical instrument isadjacent the anterior portion of the end of the femur, the systemdetermines that the “enlarged anterior view” should be shown. Thisdetermination is made based on the combination of the data regarding theposition of the surgical instrument and the data regarding thepreferences of the surgeon.

Since the system can account for the preferences of a surgeon, it isdesirable to store the preferences for each surgeon in a user profile.As a surgeon saves pre-set views, the data regarding those pre-set viewsare added to a user profile for the surgeon and attributed or mapped tothe exact types/steps of the procedure. Therefore, prior to performing aprocedure, the surgeon selects the appropriate user profile and the dataregarding the surgeon's preferences is loaded into the system.

In the foregoing discussion, the surgeon took the effort to input thedata regarding the preferences. Alternatively, the system may track asurgeon's usage to learn the preferences of a surgeon. This data is thenused to control the image viewed during the automatic view selectionmode.

Specifically, the system can track data that is generated during aprocedure and identify data that affects the view desired by a surgeon.One way that the system can track usage data is to identify correlationsbetween the position of the surgical instrument and input received fromthe surgeon regarding the desired view.

For example, as discussed above, a surgeon may routinely switch to anenlarged view when the surgical instrument is adjacent the anteriorportion of the femur. By tracking the data regarding the relativeposition of the surgical instrument and the input from the surgeonregarding the desired view, the system can learn that the surgeondesires to see an enlarged view when the surgical instrument is adjacentthe anterior portion of the femur. Therefore, during a procedure, whenthe user profile for the surgeon is loaded into the system, the systemautomatically displays an enlarged anterior view rather than the defaultanterior view when the surgical instrument is adjacent the anteriorportion of the femur.

The above example illustrates how the system can track data regarding asurgeon's use to automatically change the perspective/orientation of thescene, its pan, and zoom ratio of an image during automatic viewselection mode. It should be understood however, that this is just asimplified example to illustrate a few parameters that the system cantrack to learn a surgeon's preferences. In actual practice, the systemmay track data regarding any number of the various parameters that thesurgeon may control during a procedure. By tracking and processing thedata, the system can learn various data about a surgeon usage. This datais indicative of a surgeon's preferences. By storing and using thisdata, the system can learn a surgeon's preferences and use the dataregarding the preferences to control the image displayed during theAutomatic View Selection mode. The data-set representing a givensurgeon's user profile for a certain procedure can be transformed/loadedto be used for another surgeon should they desire this, and if they havepermission. This is useful in training; to provide the know-how fromexpert surgeons to novice surgeons. The preferences of which exact views(and all detailed parameter combinations) to use during a given step ofa surgical procedure can help novice surgeons start from an optimizedset of views and improve upon them to suit their individual preferences.

In case of a newer user of the system who has not defined preferences,the system will provide a default standard set, which is based onexperiments and the experience of other surgeons collected by thedevelopers of the system. As explained above, this default set can bemodified/customized/enhanced afterwards by the new user.

Bone Removal Simulation and Graphical Identification of Regions

As described previously, the present system 50 can be utilized toperform guided freehand surgery. Specifically, a virtual representationor model of a portion of a patient is provided, along with a model ofthe surgical tool to guide the surgeon during a procedure. The patientmodel may include a portion identified as tissue to be resected orotherwise operated on during a procedure. The system tracks the movementof the surgical tool 100, so that when the surgeon moves the tool, thesystem displays the movement of the tool in real time on a monitor,along with showing the removal of tissue that is resected in response tothe movement of the tool. Accordingly, the surgeon can align the toolwith the patient by aligning the model of the tool with the portion ofthe patient model identified for resection. Therefore, the surgeon canfollow the onscreen guidance to resect a portion of tissue.

The system may be configured to include various improvements to theprocessing and graphical representation of the patient model to improvethe guidance and/or assistance that the system provides to the surgeon.For instance, various regions of the patient model may be graphicallyrepresented in different ways to aide the surgeon in viewing the patientmodel. One way the system can do this is to display different portionsof the patient model in different colors. The different portions of themodel are selected and assigned the various colors based on theprocedure to be performed, as well as variables that are controllable bythe surgeon.

In one example, the system identifies various regions of a patient modelbased on a prosthetic to be implanted into a patient. The region to beresected will be determined based on the configuration of theprosthetic, variables specific to the procedure (e.g. whether theimplant is to be cemented or not), and preferences of the surgeon thatmay be input into the system or stored in a user profile. Based on theseand/or other variables, certain regions of the patient model may beidentified as tissue to be resected. Certain other regions may beidentified as tissue that may be resected; and certain other regions maybe identified as tissue that should not be resected.

By identifying the various regions of the patient model, the system canuse this information to display the patient model in a manner thatimproves the surgeon's ability to process the graphical data. Forinstance, the regions of the virtual model representing tissue thatshould be resected can be illustrated in a first color, such as green,The regions of the virtual model representing tissue that may beresected, but do not need to be resected, can be illustrated in a secondcolor, such as yellow. The regions of the patient model representingtissue that should not be resected can be illustrated in a third color,such as red. There could also be a gradient applied to the transitionsfrom the boundary of one region to the boundary of the adjacent region.The color gradient would create a gradual change in color from oneregion to the next. For instance, the boundary area between the greenregion and the yellow region may be colored to follow a color gradientfrom green to yellow. The color gradients aide the surgeon inidentifying transitions from one region to another.

Alternatively, the different regions may be represented by uniformcolors without color gradients between regions. In some instances, theuse of uniform colors rather than color gradients can create a contrastthat provides an alignment aide for the surgeon to use. The surgeon mayalign the surgical tool with a plane represented by the intersection oftwo regions. The color contrast of the different regions may create aneasy to read graphical representation that the surgeon can use foralignment.

In addition to providing alignment, the difference in color between theregions serves as an indication to the surgeon to proceed more slowly asthe tool approaches the resection boundary. When resecting a portion ofa bone a surgeon may cut more rapidly and aggressively when the cuttingtool is relatively far from the boundary of the area to be resected. Asthe surgeon approaches the boundary of the resection area, the surgeonmay slow the pace of cutting to ensure that the resection remains withinthe desired boundaries. By illustrating the different regions indifferent colors (or otherwise), the system provides a readilyidentifiable graphical display that informs the surgeon of the proximityof the surgical tool to a resection boundary.

Similarly, the system can be used to identify the proximity of thesurgical tool to sensitive anatomical structures, such as nerves,vessels, ligaments etc. The anatomical structures can be illustrated inred and the tissue proximate the structures can be identified in yellowas an indicator to the surgeon that the cutting tool is getting close tothe sensitive structure.

As discussed above, the contrasts between different representations ofdifferent regions of a patient model can be helpful to guide the surgeonin aligning the surgical instrument during a procedure. To furtherimprove the alignment of the surgical instrument with a particularplane, the graphical representation of the surgical instrument may bealtered. More specifically, the boundaries of the surgical instrumentmay be elongated along a line or along an entire plane.

For instance, in the case of a bone cutting saw, the blade is agenerally rectangular thin flat blade having a height, length andthickness or gauge. The thickness is normally the smallest of the threedimensions. When viewing the model of the blade on edge, the edge of theblade will generally look like a thick line. To improve the guidanceprovided to the surgeon, the height and length of the blade may beelongated. By elongating or enlarging the representation of the blade,the surgeon can more easily identify the plane of the blade to ensurethat the plane of the blade is aligned with the proper plane for a cut.The extensions are particularly helpful in illustrating whether thecutting blade is rotated relative to a desired plane.

The graphical extensions for the blade may be illustrated in the samecolor and style as the blade. In the present instance, the graphicalextensions have some characteristics that are similar to the surgicalinstrument, but there are one or more characteristics that differ. Forexample, in the present instance, the cutting tool may be displayed onthe screen as an opaque yellow item. The extensions may be a similarcolor but they may be semi-transparent, so that the surgeon can easilyrecognize that the extensions are related to the blade, while alsoeasily distinguishing which part represents the cutting tool and whichpart represents the extensions.

The system may provide controls to allow the surgeon to controlcharacteristics of the extensions, such as the extent of the extensions(i.e. how much elongation along the height, how much elongation alongthe length), the color of the extensions and the opacity of theextensions, as well as other characteristics. The controls also allowthe surgeon to manually turn the extensions on and off.

In addition to manually controlling the characteristics of theextensions, the characteristics of the extensions may be defined bypre-set views. The operation of pre-set views are described in detailabove. As an example of controlling the extensions using pre-set views,in a first pre-set view, the extensions may be illustrated in a certainmanner with a certain color and opacity. In a second pre-set view, theextensions may not be displayed at all. By switching between the firstand second pre-set views, the surgeon can control the display of theextensions. Further, as discussed above, the system may be configured toautomatically change between pre-set views based on the location and/ororientation of the surgical instrument. Therefore, the pre-set views canbe defined so that the extensions are automatically turned on or offdepending on the location and/or orientation of the surgical instrument.

As an alternative to illustrating the extensions as simply planarextensions, the cutting tool and/or extensions cutting blade may beillustrated as an oval on the display. The shape of the cutting bladethen depends on the angle of the cutting blade relative to the properplane. If the cutting blade is aligned properly, the cutting blade willlook similar to a line. As the cutting blade is twisted relative to theproper cutting plane, the cutting blade appears more rounded and oval.In this way, the variation between the angle of the cutting blade andthe angle of the proper cutting plane is readily apparent based on theovality of the cutting tool on the display.

The option of displaying the blade as an oval is another choice that canbe selected for a particular view. Therefore, in one view, the blade maybe illustrated without any extension. In another view, the blade may beillustrated with planar elongations. In yet a third view, the blade maybe represented as an oval. The surgeon may switch between theserepresentations within a view, or the representations may be defined inone or more pre-set views.

In the description above, different regions of the patient model areidentified and are illustrated differently to aide the surgeon during aprocedure. In addition to providing a graphical representation of thedifferent regions, the system may use the data regarding the differentregions to provide a graphical and/or audible warning to the surgeon.For instance, as the system detects the surgical tool approaching thearea proximate the resection boundary (e.g. the yellow zone), the systemmay display a graphical warning on the monitor 85 in addition toillustrating the surgical tool in the yellow zone of tissue on themodel. Alternatively, or in addition to the graphical warning, thesystem may provide an audible warning indicating that the cutting toolis approaching the desired boundary. The system may provide yet anotherwarning in the event the cutting tool is detected at or beyond thedesired boundary. In other words, if the surgical tool enters the redzone the system may provide a further warning.

In addition to providing warnings, the system may be configured tocontrol the operation of the surgical tool in response to the positionof the surgical tool relative to the desired boundary. Specifically, ifthe system determines that the tool is positioned within the tissue tobe resected (e.g. in the green zone), the system may allow the surgicaltool to be controlled as desired by the surgeon. If the systemdetermines that the tool is positioned within the tissue that may beresected but is near the tissue that is identified as tissue that shouldnot be resected (e.g. the yellow zone), the system may reduce orattenuate the operation of the surgical tool. For instance, if the toolis a saw, and it enters the yellow zone, the system may slow down thereciprocation of the saw as it moves close to the resection boundary.Further still, if the system detects that the tool is positioned at theboundary or in tissue that is not to be resected, the system maycompletely stop the tool. Although the system may automatically controlthe operation of the surgical tool, the system includes an overridefunction that allows the surgeon to override the control of the tool. Inthis way, if the surgeon determines that a portion of tissue should beresected that was not previously identified for resection, the surgeoncan override the system and resect the tissue during the procedure.

In the discussion above, the system controls the display of variousregions based on an identification of the regions prior to a procedure.The system may also allow regions to be identified and/or re-assessedduring a procedure. For instance, as described in the section regardingassessment of bone preparation, the system may include features relatingto assessing the cuts made during a procedure to determine whetherfurther cuts need to be made. In response to the assessment, the systemmay determine which portions of the bone need to be cut to correct aninaccuracy in a resection procedure. This correction assessment maycause a change in the regions of the patient to be resected, which willcause a change in the identification of various portions of the patientmodel. For example, a region that was initially identified as a yellowregion may be changed to a green region after the correction assessment.In this way, the identification of regions of a patient model may changeduring the course of a procedure.

In the foregoing description the operation of the surgical instrument iscontrolled based on the region in which the instrument is operating.Additionally, the operation of the surgical instrument may be controlledbased on how far into a region the instrument has traveled. Forinstance, the surgical instrument may not need to be significantlyattenuated when it just enters the yellow region. However, theinstrument may need to be significantly attenuated if it advancessignificantly through the yellow region toward the red region.Accordingly, the magnitude of control may relate to both the region thatthe instrument is positioned within and the degree to which theinstrument extends into the region.

Navigated Marking Pen

As described previously, the OR computer 80 may display a virtual modelof the portion of the patient on which the procedure is to be performed.In the discussions above, the patient model was utilized to guide thesurgeon in manipulating surgical tool to perform a procedure. Thefollowing discussion describes how the patient model can be used toguide the surgeon in preparing the site for a procedure. Specifically,the system may guide the surgeon in marking lines on the patient toidentify cut lines etc. for a procedure. The markings on the patient canthen be used alone or in combination with the guided freehand proceduredescribed above.

Referring to FIG. 13 , a navigated marking pen 250 is illustrated. Themarking pen 250 includes one or more elements for detecting the positionand orientation of the marker. For instance, the marking pen may includea reference frame 257 and a plurality of position markers 255 similar tothe frame 107 and position markers 105 described above in connectionwith the surgical tool. The marking pen 250 can be guided by viewing thedisplay of the OR computer 80 as described above in connection withoperation of the surgical tool 100. The marking pen 250 is guided todraw lines on the bone at the appropriate locations as identified on thevirtual model.

The method for using the navigable marking pen 250 operates as follows.Prior to the procedure, a virtual model of the relevant portion of thepatient is created as discussed above. The surgeon analyzes the patientmodel to determine the procedure to be performed and identifies theportion of the patient to be resected or otherwise operated upon duringthe procedure. For instance, in the instance of implanting a prostheticdevice, a femoral prosthetic may be implanted. The surgeon selects theappropriate prosthetic and aligns a model of the prosthetic over theoperation site. Based on the model of the prosthetic and the alignment,the Pre-op computer 70 may identify the tissue to be resected during theprocedure. Prior to the procedure, the patient is registered asdescribed previously, so that the patient position corresponds to thepatient model.

The OR computer 80 displays the patient model, identifying the tissue tobe resected, and the position of the marking pen is also illustrated onthe display. As the surgeon manipulates the marking pen 250, theposition detection device 120 detects the movement of the marking penand provides data to the OR computer so that the model of the markingpen moves on the screen relative to the patient model in real time.Accordingly, the surgeon manipulates the marking pen so that the modelof the marking pen aligns with the portion of the virtual modelindicated for resection.

The surgeon manipulates the marking pen 250 so that the model of themarking pen traces the area of the virtual model identified forresection or other procedure (such as drilling). In this way, thevirtual model provides a guide for guiding the surgeon to mark theappropriate areas on the patient on which the procedure is to beperformed. The surgeon may then simply perform the procedure freehandusing the markings on the patient as a guide or the surgeon may performthe procedure using the markings and also using freehand navigationassistance as described above.

FIG. 13 also illustrates another potential improvement, in that themarking pen 250 may include a retractable pen that retracts when themarking pen is not aligned with the proper area on the patient. Byretracting, it is much less likely that the surgeon may mark anincorrect area.

As shown in FIG. 13 , the marking pen 250 includes a hollow housing 260having a generally open forward end. A displaceable pen 275 is disposedwithin the hollow housing 260. The pen is displaceable between anextended position and a retracted position. In the extended position thetip of the pen extends from the housing so that the tip of the pen canbe used to mark a surface. In the retracted position the pen isretracted into the housing so that the forward tip of the pen is withinthe housing so that the pen cannot mark a surface.

A spring 285 connected to the pen 275 biases the pen toward theretracted position. An actuator 280, such as a solenoid is operable toextend the pen forwardly against the bias of the spring. Specifically,when the solenoid is energized, the solenoid drives the pen to theextended position. When the solenoid is de-energized, the spring 285retracts the pen into the housing. Alternatively, the solenoid can beconfigured to drive the pen in both directions, i.e. the solenoid candrive the pen forwardly and rearwardly as desired.

The marking pen 250 is in communication with the OR computer 80 toreceive signals indicating whether the pen 275 should be extended orretracted. The marking pen may include a wired connection to the ORcomputer, however, in the present instance, the OR computer 80 includesa transmitter, and the marking pen includes a wireless receiver forreceiving signals from the computer. The marking pen 250 includes aprocessor 270 for receiving the signals from the computer andcontrolling the extension and retraction of the pen 275 in response tothe signals. Specifically, the processor 270 controls the operation ofthe solenoid to selectively energize and de-energize the solenoid inresponse to signals received from the OR computer.

The operation of the retractable marking pen 250 is similar to theoperation described above. However, the OR computer correlates the datafrom the virtual model with the data regarding the position of themarking pen. If the OR computer determines that the marking pen ispositioned over a portion of the patient that should be marked, thecomputer transmits a signal to the marking pen 250 indicating that thepen should be extended. The marking pen receives the signal and theprocessor 270 controls the solenoid, thereby energizing the solenoid toextend the pen tip 275. If the OR computer determines that the markingpen is positioned over a portion of the patient that is not to bemarked, the computer transmits a signal to the marking pen indicatingthat the pen should be retracted and the processor controls the solenoidto retract the pen. Alternatively, the processor may be configured sothat the solenoid is energized only as long as the controller receives asignal indicating that the pen should be extended. In this way, the ORcomputer sends a signal to the marking pen as long as the computerdetermines that the marking pen is over a portion to be marked. As soonas the computer determines that the marker is over an area that is notto be marked, the computer ceases sending a signal to the marking pen.The processor then de-energizes the solenoid to retract the pen inresponse to the lack of signal.

As an alternative to a retractable tip, the marker may use an inkjet,rather than a regular marker tip. Rather than controlling the extensionor retraction of the marker tip, the ejection of the ink through theinkjet is controlled. Specifically, when the marker is over a portion ofthe portion of the patient to be marked, the marker may be enable sothat ink may flow through the inkjet. However, when the marker is over aportion of the patient that is not to be marked, the marker is disabled,so that the flow of ink to the inkjet is shut off.

As can be seen from the foregoing, the marking pen 250 can provide anaccurate and efficient method for marking cut lines and other markinglines for performing a procedure. Prior to the procedure, the surgeonmay utilize the guidance system to manipulate the marking pen byaligning the model of the pen with the area of the virtual model to beoperated on. While the surgeon maintains alignment of the virtual penwith the portions of the model indicated as proper marking lines (suchas the outline of a prosthetic), the OR computer sends a signal to themarking pen indicating that the pen element 275 should be extended. Asthe surgeon maintains the virtual pen aligned on proper parts of thevirtual model, the marking pen 250 marks the patient. If the surgeonmanipulates the pen so that the virtual pen moves out of alignment withthe proper parts of the virtual model, the OR computer sends a signal tothe marking pen (or ceases sending a signal to the pen as describedabove), and the pen tip 275 retracts into the housing so that the penstops marking the patient. In this way, the surgeon controls theretraction of the pen by maintaining alignment of the virtual pen withthe portion or portions of the model that were identified as portions tobe marked.

Registration Pointer with Surface Contact Detection

Prior to or during a procedure, a digitizing pen or pointer may be usedto identify the location of points of reference on a patient. Similarly,the pointer may be used to mark areas to identify a surface. The markedpoints identify the location of discrete points or areas on a patient tocorrelate or register the patient model with the actual position of thepatient.

Although a pointer can be helpful in registering the location of apatient, human error can lead to errors in properly registering thepatient location. For instance, when the surgeon is tracing the surfaceof the patient tissue, the tip of the pointer may come out of contactwith the surface of the tissue. This is particularly true when tracingover soft tissue or when tracing along curved surfaces. If the pointeris not in contact with the surface of the tissue, the resulting datapoints will be erroneous.

To improve the accuracy of the data collected during registration, thesystem may include a pointer that incorporates a sensor that detectswhether the pointer is in contact with the patient tissue. If thepointer is out of contact with the surface of the relevant portion ofthe patient, the points are ignored during the registration analysis.Additionally, the system may provide feedback to the surgeon to warn thesurgeon that the point is out of contact with the patient tissue.

Referring to FIG. 15 , an improved registration pointer is designated350. The pointer is an elongated element having a tip configured tocontact the relevant portion of a patient. The pointer 350 isoperatively linked with the position detection device 120. The operativelink may be a wireless connection in which the pointer includes awireless transmitter. Alternatively, the pointer may be connecteddirectly to the detection device via a cable.

The pointer includes a sensor 360 for detecting whether the tip of thepointer is in engagement with the patient or whether the tip of thepointer is spaced apart from the patient. One possible sensor 360 is animpedance sensor. Alternatively, the sensor may be a simple forcetransducer. The pointer 350 includes a circuit 365 for analyzing thesignal from the sensor and determining whether the pointer is in contactwith the patient surface based on the signal from the sensor.

The data for the point or points in which the pointer was out of contactwith the patient surface are not utilized during the registrationprocess. Specifically, the pointer circuit may identify valid andinvalid data by various means. According to a first method, the pointercommunicates the relevant data to the OR computer 80 via a wired orwireless connection. Alternatively, the pointer circuit may control theposition tracking elements so that the pointer is out of view of theposition detection device 120 when the pointer 350 is out of contactwith the patient surface.

According to the first method, the OR computer receives signals fromboth the pointer 350 and the position detection device 120 and processesthe data. The pointer circuit provides a signal to the OR computerindicating whether the pointer is in contact with the patient tissue.The OR computer 80 receives the signals from the pointer circuit 365along with signals from the position detection device 120 that indicatethe position of the pointer. Based on the signal received from thepointer 350, the OR computer 80 either accepts or rejects the positiondata received from the position detection device 120. For instance, ifthe surgeon is tracing the pointer over an area of tissue, the ORcomputer will accept the position data regarding the area traced by thepointer as long as the sensor 360 detects that the pointer tip is incontact with the patient. If the sensor 360 detects that the pointer isout of contact, the OR computer discards or rejects the data from theposition detection device 120 corresponding to the positions that thesensor detected the pointer is out of contact. In this way, as long asthe pointer remains out of contact with the patient surface, the ORcomputer discards the corresponding position data from the positiondetection device.

The system may be configured to process the signals from the pointer 350and the position detection device 120 in a variety of ways. Forinstance, the OR computer may reject data from the position detectiondevice unless the pointer sensor 360 provides a signal indicating thatthe pointer is in contact with the patient tissue. Alternatively, the ORcomputer may accept data from the position detection device unless thepointer sensor 360 provides a signal indicating that the pointer is outof contact with the patient tissue. In either alternative, the ORcomputer only records data for points in which the pointer is in contactwith the patient tissue.

In an alternative embodiment, the OR computer does not reject the datato eliminate erroneous data. Instead, the system alters the positiondetection device 120 to prevent the erroneous points from beingdetecting.

Specifically, the system controls features of the position detectiondevice 120 to essentially make the pointer disappear from view of theposition detection device when the pointer is out of contact. Since thepointer is out of view when it is out of contact with the patient, nodata is collected while the pointer is out of contact.

The steps for rendering the position detection elements out of view ofthe detector varies depend on the type of detection element. Forinstance, as described previously, the position detection device mayoperate in conjunction with passive and active markers. An active markeris a marker that transmits an infrared signal to the detection deviceand the position of the marker is identified by triangulating thereceived signal. Accordingly, to control the active marker(s), thepointer circuit 365 controls the active markers by turning off theactive markers so that they no longer emit an infrared signal when thepointer is out of contact with the relevant portion of the patient. Whenthe emitter ceases emitting infrared light, the marker is hidden fromthe position detection device 120 so that the registration points arenot detected.

If the markers on the pointer are passive elements, the markers aredetected by detecting the infrared light reflected back to the positiondetection device 120. In order to hide such passive markers the pointercircuit may be used to control one or more elements including adisplaceable opaque surface and an electronically/chromatically actuatedeffect to disable the infra-red reflectivity of the ball. Accordingly,for both passive marker systems and active marker systems, the systemmay control the position detection device 120 in response to signalsfrom the pointer 350 indicating that the pointer is out of contact withthe patient tissue.

In addition to controlling whether or not data points are accepted orrejected, the system may provide feedback to the surgeon warning thatthe pointer is out of contact with the patient tissue. For instance, ifthe sensor 360 on the pointer 350 indicates that the pointer is out ofcontact with the patient tissue, the pointer circuit 365 may provide asignal to an indicator light, such as an LED on the pointer. Therefore,if the surgeon sees the LED illuminated, the surgeon will recognize thatthe pointer needs to be pressed against the patient. Alternatively, thesignal from the sensor circuit can be communicated with the OR computer,so that a warning is displayed on the display screen. In addition toproviding a visual warning, the system may provide an audible warning.Specifically, the pointer circuit 365 may provide a signal to an audioelement to provide a warning signal, such as a beep when the pointer isout of contact with the patient.

There may be instances in which the pointer is out of contact with thepatient so often that it may be desirable to re-start the registrationprocess. Accordingly, the system may track the number of times that thesensor 360 detects that the pointer is out of contact with the patient.If the number of times exceeds a pre-set threshold, the system may senda warning to the surgeon indicating that the registration process shouldbe re-started.

Combination Cutting and Filing Blade

Referring to FIGS. 18-19 an alternate cutting blade 102′ is illustrated.The cutting blade 102′ includes both an edge that can be used forcutting, as well as a surface that can be used for filing. Specifically,the cutting blade 102′ has a cutting edge with a plurality of cuttingteeth 103. The teeth are formed in a row and may have a set. The row ofteeth are operable to cut when the blade is reciprocated. The blade alsoincludes a surface that can be used for filing, such as filing a bonesurface during a procedure.

As shown in FIGS. 18-19 , the blade has two sides, A and B. Side A is agenerally smooth surface. Side B is on the side opposite from side A,and is formed with a plurality of cutting surfaces that form a filingsurface. Specifically, side B is a generally elongated planar surfacehaving a length and a width. A plurality of spaced apart ridges or teeth101 protrude upwardly from the surface of the blade. Each ridge extendsacross the width of the blade so that the ends of each ridge terminateat the edges of the blade. However, the ridges 101 need not extendacross the entire width of the blade.

The ridges 101 on side B of the blade 102′ form a secondary cuttingsurface that can be used for different procedures than the row ofcutting teeth 103. Specifically, the row of teeth 103 can be used tomake a cut in a bone. In contrast, the ridges 101 form a filing surfacethat can be used to file a bone surface.

Further, the cutting blade 102′ can be used in a computer assistedtechnique, similar to those described above. The row of teeth can beused in a computer guided procedure in which the computer aligns thecutting blade to cut a portion of bone. Similarly, the filing teeth canbe guided using a computer guided procedure in which the computer guidesthe filing teeth to remove a portion of bone that is identified forremoval.

Tool Registration Head

One step during navigated surgery is the registration of the surgicaltools. Registration is the process of identifying the specific locationand orientation of a surgical tool. If a tool is not properly registeredthe navigation of the tool will be flawed leading to errors during theprocedure.

As discussed previously, the system may provide guidance by displaying agraphical illustration of the position of the surgical instrumentrelative to the patient. The system provides this guidance by trackingthe location and orientation of the surgical instrument. For example, inthe present instance, the surgical instrument 100 includes a framehaving a plurality of markers 105 that are used to track the positionand orientation of the surgical instrument. By tracking the location andorientation of the reference frame, the system can track the locationand orientation of the surgical instrument. Since the structure of thesurgical instrument is generally fixed relative to the reference frame,identifying the location and orientation of the reference frame can beused to identify the location and orientation of the surgicalinstrument.

Although the configuration and dimensions of the surgical instrument aregenerally fixed, the surgical instrument may be used with one or more ofa variety of different cutting tools or accessories during a procedure.For instance, the surgical instrument may use any of a number ofdifferent sized saw blades or drill bits. Since the tool is typicallythe part of the surgical instrument that actually operates on thepatient tissue, it is important to accurately identify the configurationof the tool, as well as the location and orientation of the tool.

To properly track the position of the tool during a procedure, theregistration process identifies the configuration of the tool, as wellas the location and orientation of the tool relative to the positiondetection element(s) on the surgical instrument. Specifically, theregistration process identifies the position and orientation of the toolrelative to the frame and markers 105 on the surgical instrument 100.

Referring to FIG. 14 , a tool registration head 300 is illustrated. Theregistration head 300 is designed to quickly and easily register a toolso that the system can accurately track the position and orientation ofthe tool. In this way, the registration head allows a surgeon to changetools during a procedure without the undue delay involved in knownprocesses for registering a tool.

In the following discussion, the registration head 300 is described inconnection with registering a variety of cutting tools. However, itshould be understood that this is an example of one type of tools thatthe registration head may be configured to register. The registrationhead can be configured to register any of a variety of cutting toolsand/or accessories that can be used in the surgical instrument.

The registration head 300 includes a plurality of sockets that areconfigured to mate with a plurality of cutting tools that can be used inthe surgical instrument. The sockets are configured so that each socketcooperates with a particular tool. In this way, the system identifiesthe tool type in response to a determination of the socket into whichthe tool is mounted. In the present instance, the registration head 300includes three rows of sockets. The first row 312 a,b,c is configured toregister drill bits. The second row 314 a,b,c is configured to registersaw blades; and the third row 316 a,b is configured to register othertools.

The first row includes three sockets, 312 a, 312 b and 312 c. Eachsocket is cylindrical having a different diameter. In this way,inserting a tool in the first socket 312 a registers the tool as a drillbit having a particular diameter, inserting the tool in the secondsocket 312 b registers the tool as a drill bit having a particulardiameter, which in the present instance is larger than the diameter ofthe first socket. As can be seen, the registration block may have aseries of numerous cylindrical sockets for registering numerousdifferent drill bits. Each socket would have a different diameter forregistering a particular diameter drill bit.

The second row of sockets in the registration head 300 is configured toregister saw blades, such as sagittal blades. Such blades are generallythin flat blades having a row of teeth on one end. The second row ofsockets includes a first socket 314 a in the form of a rectangular slothaving a height and width. The height corresponds to the thickness of aparticular saw blade and the width corresponds to the width of the sawblade. Therefore, inserting a tool into the first saw blade socket 314 aregisters the tool as a sagittal blade having a predetermined width andthickness. Similarly, inserting a tool into the second saw blade socket314 b registers the tool as a sagittal blade having a predeterminedwidth and thickness that is larger than the blade corresponding to thefirst saw blade socket 314 a. As with the sockets for the drill bits, itshould be appreciated that the registration block can include a numberof different sized slots configured to mate with numerous different sawblades. Each slot would be configured to mate with a specific saw bladeso that the blade can be uniquely identified and registered by simplyinserting the blade into the appropriate slot.

In addition to the sockets, the registration head 300 also includes aplurality of position detection elements 310. The position detectionelements may be either passive marker elements or active markerelements, as discussed previously. The type of position detectionelements is selected to cooperate with the tracking system 120 that isused. For instance, in the present instance, the registration head 300includes a plurality of spaced apart spherical reflective markers 310.

The spacing and the orientation of the position detection elements 310are known. Therefore, the tracking system 120 can determine the positionand orientation of the registration head 300 by detecting the locationof each position detection element 310. Additionally, the positionand/or configuration of the registration sockets in the head 300 isknown.

Accordingly, as described above, the tracking system 120 can track theposition detection elements 310 on the registration head 300 todetermine the position and orientation of the registration block.Similarly, the tracking system 120 can track the position detectionelements 105 on the surgical instrument 100 to determine the positionand orientation of the surgical instrument. Since each socket in theregistration head defines a unique location for a particular tool, theposition of the surgical instrument relative to the registration blockis unique for each particular tool. The unique spacial relationshipsbetween the registration block and the surgical instrument ispredetermined for each tool that the registration block is configured toregister.

Part of the information that is determined during the registrationprocess is the position of the tip of the tool relative to the positiondetection elements 105 on the surgical instrument. As discussed above,the configuration and orientation of each tool relative to the surgicalinstrument can be determined depending upon the socket into which thetool is inserted. However, this process does not necessarily identifythe position of the tip of the tool. For instance, if a tool isregistered when the tool is inserted only halfway into a socket, thesystem will incorrectly assume that the tip of the tool is at a positioncorresponding to where the tip would be if the tool was fully insertedinto the socket.

To properly identify the location of the tip of a tool, each socket hasa bottom wall that acts as a stop. The location of each bottom wall is afixed location relative to the position detection elements 310, which ispre-determined for the registration head. Since it is a fixed and knownlocation, the bottom wall operates as the assumed location of the tip ofthe tool when a tool is inserted into a socket. Therefore, to properlyregister a tool, the tool is inserted into a socket until the tip of thetool engages the bottom wall of the socket.

Based on the foregoing, a tool, such as a saw blade can be easilyregistered by simply attaching the tool to the surgical instrument 100and then inserting the tool into the proper socket in the registrationhead 310. The tool is fully inserted into the socket until the tip ofthe tool engages the bottom of the socket. During the registrationprocess, the tracking system 120 tracks the position and orientation ofthe registration block relative to the surgical instrument, whichidentifies the configuration and orientation of the tool relative to theposition tracking elements 105 on the surgical instrument. For instance,in the case of a saw blade, if the blade is inserted into slot 314 a,the relative position between the registration block and the surgicalinstrument identifies the tool as a saw blade having a particular heightand width. Furthermore, the saw blade fits into the slot in a particularorientation, so the orientation of the blade is also know.

In other words, the tracking system 120 tracks the position detectionelements 310 to track the position and orientation of the registrationhead. The tracking system also tracks the position detection elements105 on the surgical instrument to determine the position and orientationof the surgical instrument. As discussed previously, the spacialorientation between the surgical instrument and the registration blockis pre-determined and unique for each socket. Therefore, when a tool isinserted into a socket in the registration head, the spacial orientationbetween the surgical instrument and the registration head 300 definesthe boundaries of the tool relative to the tracking elements 105 on thesurgical tool. Accordingly, after the tool is registered, the system canaccurately track the boundaries of the tool mounted in the surgicalinstrument by tracking the position of the tracking elements 105 on thesurgical instrument.

The process for registering a tool may be either manual or automatic. Ina manual mode, the tool is inserted into the proper socket until the tipof the tool contacts the bottom wall of the socket. While the tip is incontact with the bottom wall of the socket, the surgeon presses a buttonto indicate that the tool is properly registered in a socket. The systemuses the positional data from the tracking system 120 at the time whenthe button was pressed to determine the position of the trackingelements 310 on the registration head 300 and the position of thetracking elements 105 on the surgical instrument 100. Based on theposition data, the system calculates the location of the boundaries ofthe tool relative to the tracking elements 105 on the surgicalinstrument.

In addition to pressing the button, the indicator signal for registeringa tool can be other various forms of inputs. For instance, the inputsignal could be a voice signal that is recognized be voice recognitionsoftware. Alternatively, the input signal could be an area on a touchscreen, a click of a mouse, an input mechanism on the surgicalinstrument itself, a keyboard stroke or otherwise.

In an automatic mode the surgeon need not press a button to indicatethat the tool is inserted into the registration head 300. Instead, thesystem automatically determines that the tool is inserted into a socketand makes the registration calculations.

One automatic mode relies upon sensors in the registration head 300.Specifically, a sensor is located in each socket of the registrationhead. The sensors may be any of a variety of types of sensors, such asan impedance sensor or a load transducer. The sensor detects whether atool is in contact with the bottom wall of the socket. When the sensordetects that the tool is in contact with the bottom wall, the sensorsends a signal to the OR computer or the tracking system 120. The signalfrom the sensor operates similar to the surgeon pressing the button inthe manual mode described above.

A second automatic mode may be used instead of the first automatic modedescribed above. The second automatic mode automatically registers thetool without using sensors in the registration head. In the second mode,the tool is inserted into the registration head 300 and is held in theappropriate socket for a pre-determined time period, such as 1 or 2seconds. The system tracks the location of the surgical instrumentrelative to the registration block and registers the position of thetool when the surgical instrument is at a fixed position for thepre-determined time period. In this way, the period of time that thesurgical instrument is stationary relative to the registration blockoperates as the trigger for registering the tool.

Although the second automatic mode may use a hold time as the trigger,the system may need to ignore positions that do not correspond to validregistration positions. For instance, if the surgical instrument isstationary on a surface sitting next to the registration head, the toolwill be stationary for a sufficient period to trigger registration.However, when the surgical instrument is sitting next to theregistration head, the position of the surgical instrument relative tothe registration block does not correspond to a valid registrationposition. Therefore, in the second automatic mode, the system may rejectregistration data corresponding to positions that are invalidregistration positions.

In another alternative mode, rather than rely on a hold time as thetrigger, the system may simply evaluate the positional data to registera tool. Specifically, the system may monitor the position of thesurgical instrument relative to the registration head to determine whichof the sockets the tool is inserted into, as described above. As thetool is inserted into the socket, the surgical instrument will move in aparticular direction. For instance, in the example of registering adrill bit, the surgical instrument will move towards the registrationblock along an axis as the drill bit is inserted into the correspondingsocket. Assuming that the surgeon inserts the tool until it touches thebottom wall of the socket, the registration position will relate to themaximum point of travel along the axis. As with the second mode above,the system will ignore data that does not correspond to a validorientation of having a tool inserted into one of the sockets.

Once the tool is registered, the system tracks the boundaries of thetool by tracking the location of the tracking elements 105 on thesurgical instrument. If the surgeon desires to use a different toolduring a procedure, the surgeon simply needs to replace the tool withthe new tool, indicate to the system that a new tool is to beregistered, such as by pressing a button or otherwise, and theninserting the new tool into the appropriate socket in the registrationblock as described above. Since the registration process is quick andeasy, the overall procedure is not significantly delayed when a new toolis needed.

As discussed above, the positional data regarding the dimensionalconfiguration of the registration block is pre-determined, as is dataregarding the positional relationship between the surgical instrumentand the registration block for each socket. This data is stored ineither a file on the OR computer or the position detection system 120.Similarly, data regarding each type of tool that correlates to eachsocket may be stored in a data file on the OR computer.

Assessing and Correcting Bone Cuts

When implanting a prosthetic onto a bone, the surgeon must resectportions of the bone to prepare the bone to receive the prosthetic.Regardless of how the resection is performed, it is helpful to assessthe quality of the cuts performed during a procedure prior implantingthe prosthetic. Bad fit between the bone and the prosthetic causes asignificant number of implant failures. Therefore, a close match betweenthe shape and dimensions of the prepared bone and the prosthetic isimportant to the proper affixation and durability of the implant. Thesurgeon may rely upon experience and trial and error during a procedure,however, doing so does not provide a quantifiable method for ensuringthat a resection is proper.

Accordingly, it may be desirable to incorporate a method and apparatusfor assessing the quality of bone cuts before a prosthetic is implanted.Additionally, after assessing the bone cuts, it may be desirable toprovide feedback regarding any additional shaping that should be made toimprove the bone cuts to prepare the bone to receive the implant.

The assessment of bone cuts evaluates four aspects: surface finish, fit,alignment and accuracy. The surface finish relates to the smoothness ofthe cut surfaces. The fit relates to how closely the bone shape matchesthe shape of the implant. The alignment relates to whether or not thecuts are made so that the implant is positioned in the proper threerotations of alignment, including the flexion/extension axis, theanterior/posterior axis, and the proximal/distal axis. The accuracyrelates to the angle and orientation of each cut relative to the planeof the ideal cut.

During a procedure, bone is resected according to the geometry of theprosthetic to be implanted. After the resection, the cut bone isanalyzed to evaluate the four aspects mentioned above: surface finish,fit, alignment, and accuracy.

Referring to FIG. 1 , the system for assessing the bone cut comprises ascanning device 320 that communicates with a processor, such as apersonal computer, which may be the OR computer 80. The processorcommunicates with an output device, such as a monitor 85 to illustrateinformation about the assessment of the bone cuts.

The scanning device 320 may be one of a number of various devices foracquiring information. For instance, the scanning device 320 may be aprobe such as the smart probe discussed above. The probe is traced overthe cut surfaces to obtain data regarding each surface.

Analysis of Surface Finish

The processor analyzes the scanned data to evaluate each cut of theresected bone. For instance, as shown in FIG. 10 , in the case of a TKRprocedure, there are typically five separate cuts made to the femur whenthe bone is resected to accommodate the prosthetic (it may be consideredseven cuts rather than five when considering the posterior condyleresection as two cuts, as well as the posterior chamfer). The image datafor the resected bone is analyzed to assess the surface finish for eachof the five cuts.

The analysis of surface finish may include an analysis of one or morecharacteristics to evaluate whether the surface is of sufficient qualityto bond well with the prosthetic. In the present instance, the systemanalyzes the roughness and/or the waviness of the resected surface toassess the surface finish. Roughness includes the finer irregularitiesof a surface that generally result from a particular cutting tool andmaterial conditions. Waviness includes the more widely spaced deviationof a surface from the nominal or ideal shape of the surface. Waviness isusually produced by instabilities, such as blade bending, or bydeliberate actions during the cutting process. As illustrated in FIG. 7, waviness has a longer wavelength than roughness, which is superimposedon the waviness.

Based on analysis of the 3D geometrical image data, the surface finishfor each cut is analyzed and quantified. In the present instance, thesurface finish may be quantified based on: (1) the roughness average,(2) an average of the heights of a select number of the worst peaks(i.e. highest surface peak relative to ideal surface); (3) an average ofthe heights of a select number of the worst valleys (i.e. deepest valleyrelative to ideal surface); and (4) a measure of the deviation from theaverage height of the worst peaks and the average depth of the worstvalley (i.e. (2)-(3)). In some instances, it may be desirable toseparate the quantification of the measure of waviness from the measureof roughness. However, in the present instance, roughness and wavinessare evaluated together. An example of a resected femur havingunacceptable surface finish is illustrated in FIG. 8 . As can be seen,the geometry of the resection is proper, so that the prosthetic wouldfit properly onto the resected bone and be properly aligned. However,due to the poor surface finish it is likely that the bond between thebone and the prosthetic will fail prematurely. Based on the surfacefinish analysis, the surgeon may decide that one or more of the cuts mayneed to be smoothed, such as by filing.

Analysis of Fit

The second characteristic evaluated for the bone cuts is fit. Fitrepresents the looseness or play between the implant and the resectedbone shape prior to affixing the prosthetic to the bone. An example of aresected femur having an unacceptable fit error is illustrated in FIG. 8. As can be seen, the surface of each cut is acceptable and theorientation of each cut is acceptable, however, the resultant shapeleaves unacceptable gaps between the prosthetic and the resected bone.The gaps create play or looseness that will lead to misalignment and/orpremature failure of the bond between the bone and the prosthetic.

To measure the fit error, a fitness measuring block 340 may be utilized.The fitness measuring block 340 is a block having an internal shapecorresponding to the internal shape of the prosthetic (i.e. the surfacethat will bond with the bone). A tracking element 345 for detectingdisplacement is attached to the fitness measuring block. In the presentinstance, the tracking element is an infrared tracking device similar tothe frame and markers 105 that are used to track the surgicalinstrument, as described above. Alternatively, a navigated implant trialthat is specific to each prosthetic implant may be used rather than ameasuring block. The navigated implant trial is an implant similar tothe prosthetic that is to be implanted into the patient. The navigatedimplant would also include an element for detecting the position of theimplant trial, such as the tracking element 345 described above. Thetracking system 20 (see FIG. 1 ) tracks the position of the trackingelement 345 and communicates data to the processor that is indicative ofdisplacement of the measuring block 340 relative to the resected bone.

To use the measuring block 340, the block is placed over the resectedbone. The surgeon then attempts to move the measuring block in alldirections relative to the bone to evaluate translational error based onthe amount of translation possible between the measuring block and theresected bone. Specifically, the surgeon rotates the block in flexionand extension, as well as internally and externally. In other words, thesurgeon rotates the blocks about several axis relative to the bone, suchas an axis running generally parallel to the axis of the bone (i.e.rotation internally and externally) as well as an axis running generallytransverse the axis of the bone (i.e. rotation in flexion andextension).

As the surgeon moves the measuring block, the tracking system 120 tracksthe translational and rotational movement of the measuring blockrelative to the bone. The tracking system communicates the dataregarding the movement of the measuring block with the OR computer 80.Based on the data from the tracking system 120, the OR computer analyzesand quantifies the fit based on the measured translational error and themeasured rotational error. Specifically, the OR computer 80 analyzes thedata regarding movement of the measuring block to measure extremes ofmovement of the measuring block relative to the bone. The extremes ofmovement are measured in each of six direction of movement/rotation. Theextremes of motion indicate the looseness of the fit. If the measuringblock can move significantly relative to the bone, then the extremes ofmovement will be significant. This will reflect a loose fit. Conversely,if the measuring block cannot move significantly relative to the bone,then the extremes will be relatively small. This will reflect a tightfit.

Analysis of Alignment

A third characteristic for assessing the cuts is the alignment of thecuts. The alignment assessment can be performed using the same data setthat was collected while analyzing the implant fit as described above.

The alignment error is a quantification of the deviation of the locationof the measuring block from the ideal location at which the implants areto be positioned for proper alignment. Specifically, in the presentinstance, the alignment error is based on three rotational deviationsand three translational deviations from the ideal locations.

The deviations of the measuring block from the ideal position(s) arebased upon the data obtained by the tracking system 120 during trackingof the measuring block to evaluate the fit, described above. The ORcomputer 80 analyzes the data regarding the movement of the measuringblock relative to the resected bone. The OR computer 80 analyzes thedata to evaluate whether the measuring block passed through the idealalignment position as the block was moved between the extremes ofmovement.

If the measuring block passes through the ideal alignment during thetest manipulation, the alignment is correct; if not, the alignment isoff. To evaluate the alignment error, the computer analyzes the datafrom the tracking system to determine how close the measuring block cameto the proper alignment position. This deviation is analyzed for the sixparameters about the three rotational axes mentioned above.

In other words, for each axis of rotation, the system determines theposition that the measuring block should pass through to be ideallyaligned in the axis of rotation. The system determines how close themeasuring block came to the ideal position when the block was rotatedalong the same axis of rotation. The deviation is analyzed for each axisto obtain an overall error measurement.

Analysis of Accuracy

A fourth characteristic used in the present instance to evaluate thebone cuts is the accuracy of each cut. For example, in the instance of aTKR procedure, the accuracy of each cut is evaluated. The importance ofthe accuracy of the cuts is exemplified by the third sample illustratedin FIG. 8 . As can be seen, the sample has acceptable surface finish,fit and location. In other words, the prosthetic will fit well on thebone (i.e. it won't wiggle excessively), the surface finish is not toorough or wavy and the prosthetic will be properly aligned with the bone.However, due to the inaccuracy in one or more of the cuts, there will begaps between the prosthetic and the bone that will increase thelikelihood of premature failure.

To evaluate the accuracy of the cuts, the deviation between the actualcuts and the ideal cuts for the particular prosthetic is measured. Theideal cuts are determined based on the geometry of the prosthetic to beimplanted on the resected bone. For instance, in the example of a TKR,the ideal cuts for the femur are based on the internal configuration ofthe femoral prosthetic. One way of determining the ideal cuts is tocreate a model of the configuration of the ideal cuts for the patient.

In the present instance, a scanner can be used to create a threedimensional model of the resected bone, as discussed further below. Thedata obtained from the scanner for each planar resected surface iscompared with the data for the corresponding surface of the idealresected model to evaluate the accuracy of the cuts. The quantificationof the accuracy can be based on a variety of measurements regarding thedeviation of each resected surface from the ideal surface.

To assess accuracy, the plane of each cut is calculated using a best fitplane. The deviation of the best fit plane from the ideal plane isanalyzed to determine accuracy. Specifically, in the present instance,four characteristics are measured to assess accuracy. The firstcharacteristic is a translational measurement, and it is calculated asthe distance between the best fit plane of the resected surface and thecentroid of the corresponding ideal cut. The remaining threecharacteristics are rotational angles. The first rotationalcharacteristic is the orientation of the resected surface relative tothe ideal plane with respect to a first axis; the second rotationalcharacteristic is relative to a second axis and the third rotationalcharacteristic is relative to a third rotational axis. Thesecharacteristics are measured and correlated to quantify the accuracy ofeach planar cut of the resected bone.

Each cut is analyzed independently in the accuracy assessment.Therefore, the assessment can be used to detect which adjustments needto be made to a cut if the cut exceeds an error threshold. The systemcan suggest one or more cuts or modifications to be made based on ananalysis of the resected bone and the ideal cuts.

In the foregoing description, the evaluation of the location error andfit error are based on measurements provided by manipulating the fitmeasurement block 340 relative to the resected bone. Alternatively, thefit and location errors may be evaluated using a virtual comparison ofthe resected bone and models of ideal location and fit for the bone. Forinstance, the resected bone may be scanned to create a three dimensionalmodel of the resected bone. The scanner may use electromagnetic,ultrasonic/acoustic, mechanical, infra-red line-of site, or otherelements. For instance, a three dimensional optical laser scanner,scriber, navigated digitizer, coordinate measuring machine or CT-baseddigitization can be used to create a digital model of the bone surface.

Prior to the procedure a three dimensional model of the relevant portionof the patient can be created using any of a variety of techniques,including but not limited to CT scans and MRI images. The processor mayinclude a database of models corresponding to various prosthetics. Thesurgeon selects the appropriate prosthetic model and positions itrelative to the model of the relevant portion of the patient. Theprocessor then modifies the patient model to reflect the ideal resectedsurfaces for the selected prosthetic. Using collision detectionalgorithms, the scanned data for the resected bone can be compared withthe data for the model for the ideal resected bone to calculate thevarious criteria used to measure fit error, alignment error and/oraccuracy error.

Feedback from Assessment

After the processor determines the various criteria to assess thequality of the cuts, the information regarding the criteria may bedisplayed on the monitor to indicate to the surgeon whether or not thecuts were of sufficient quality to proceed with implanting theprosthetic on the bone. Additionally, if the cuts are not of sufficientquality, the processor may evaluate the cuts to determine a strategy formodifying the resected bone to improve the quality of the cuts. Forinstance, based on a comparison of the scanned data for a resected bonewith the data for the model of an ideal resected bone, the processor maydetermine the portion(s) of bone that should be re-shaped to improve thecorrelation between the resected bone and the model for the idealresected bone. After determining the portions of the bone that should bere-shaped, such changes are displayed on the monitor to show the surgeonwhich portion(s) of the bone should be removed. For example, using agraphical output, the bone may be illustrated generally in white and theportion(s) of the bone that should be resected to improve the fit withthe prosthetic may be shown in red.

Intraoperative Surgical Motion Recording

As discussed previously, during a procedure, the tracking system 120tracks the position and orientation of the surgical instrument 100. Thistracking data is used to provide real-time feedback and/or guidanceduring the procedure. In addition, the system may store the trackingdata for later review and/or analysis. For instance, after a procedure,a surgeon can review the stored data to see each step that was takenduring a procedure and the order in which each step was taken. Suchanalysis can provide valuable insight into advantages and/ordisadvantages of particular steps of particular procedures.

The tracking system 120 is designed to track the movement of thetracking elements 105 of the surgical instrument. The tracking system isable to track the tilt, roll, pitch and offset of the surgicalinstrument as it in manipulated. Further, the tracking system 120 isable to track the position, and orientation of the surgical instrumentand the speed of movement. Further still, the system is able to trackthe position of the surgical instrument relative to the bone.

The tracking data is stored, so that it can be re-called to review aprocedure. Specifically, the data can be re-called so that each step ofa procedure can be reviewed in the sequence that each step was performedduring the procedure.

As discussed previously, the tracking system 120 tracks the position andorientation of the surgical instrument 100 and the system correlates theposition of the instrument with the position of the patient. The systemthen displays an illustration of the position of the surgical instrumentrelative to the patient. Specifically, the system displays a model ofthe patient tissue and displays a model of the surgical instrument in aposition and orientation relative to the model corresponding to thetracked position and orientation of the instrument relative to thepatient.

Since the stored data relates to the three-dimensional position andorientation of the surgical instrument relative to the patient, the datacan be used to display a three-dimensional representation of thesurgical procedure. For instance, similar to the playback of a movie,the stored data can be used to display an illustration of each step in aprocedure in the sequence that each step occurred. Additionally, sincethe tracking data includes three-dimensional information, theillustration is not limited to the view that was displayed during theprocedure, as would be the case if a movie was taken of the procedureand then watched later. In contrast, during the review of the procedure,the tracking data can be used to watch a step of the procedure from anydesired perspective or view. For example, during a procedure, a certaincut may be guided by viewing the patient model from an anterior view.When reviewing the data stored for the procedure, the user may view thestep from a posterior view, or any other view that the user desires. Inthis way, the user may evaluate each step from any number ofperspectives to identify how a step was accomplished and/or the resultof a particular step.

The stored tracking data can be used in a variety of applications. Forinstance, a surgeon may review the tracking data to assess theefficiency or effectiveness of various procedures. Similarly, the datacan be used to teach procedures to other surgeons, such as lessexperienced surgeons. The less experienced surgeon will be able to seeexactly what steps were taken and the result of each step and the resultof the overall procedure.

In addition, to allowing the review of data from a procedure, thetracking system can be used to simulate a procedure. Using the patientmodel, the surgeon may manipulate the surgical instrument to simulatethe steps taken during a procedure. The system tracks the manipulationof the surgical instrument and illustrates the effect of eachmanipulation on the patient model. The surgeon can see the effect ofeach manipulation on the patient model in real-time. In this way, thesimulation allows a surgeon to test or evaluate a procedure on a modelof the patient. The data regarding the simulation can be recalled duringan actual procedure to guide the surgeon.

In the alternative, the stored data can be used to guide a automatedsurgical instrument, such as a surgical robot. The surgeon canmanipulate a surgical instrument to perform a simulated procedure, asdiscussed above. Once the surgeon has finalized and validated each stepin a procedure, the data regarding the position and orientation of thesurgical instrument at each step of a procedure is stored. The storeddata is then used to guide an automated surgical instrument, such as arobot, to perform the procedure. The automated instrument will followthe guidance of the stored data, so that the surgical instrument ismanipulated to track the actions of the surgical instrument during thesimulated procedure.

Anchoring Device for Surgical Tool

When performing a navigated freehand, one of the issues is maintainingthe alignment of the surgical instrument, particularly during the startof a cut. For instance, at the beginning of a cut the saw blade may tendto wander from the desired cut line or plane. However, once the cutbegins, the blade creates a kerf, which tends to limit the movement ofthe saw blade. Unfortunately, if the saw moves out of alignment at thebeginning of a cut, the saw kerf can constrain the saw blade in a planethat is out of alignment, thereby furthering the problem created whenthe saw moved at the start of the cut. To limit the misalignment thatmay occur at the beginning of a cut, the surgical tool may includes ananchoring device mounted on the surgical tool.

Referring to FIG. 4 , the anchoring device 115 is an elongated elementpositioned adjacent the cutting tool 102. The forward end of theanchoring device 115 is positioned so that the tip of the anchoringdevice protrudes beyond the tip of the cutting tool. In the presentinstance, the anchoring device 115 includes a plurality of pins orspikes 116 positioned at the end of the anchor. The pins 116 areconfigured to anchor the anchor 115 into bone, and the pins may retractafter a cut begins. The anchoring device 115 may also include a recessfor receiving the pins 116 when the pins retract so that the pins do notinterfere with the cutting operation of the tool.

Although the anchoring device 115 can be configured in a variety ofshapes, in the present instance, the anchoring device is an elongatedflat bar positioned parallel to the cutting blade 102 and in closeproximity to the cutting blade. The anchor 115 preferably is more rigidthan the cutting blade, and preferably is substantially rigid relativeto the cutting blade. In this way, the anchoring device supports thecutting tool, limiting the deflection of the cutting blade toward theanchor.

During a procedure, the anchoring device operates as follows. Asdescribed above, the surgeon views the monitor to properly align thesurgical tool to perform the cut. After the surgical instrument isaligned, the surgical instrument is anchored to the bone by driving theinstrument toward the patient bone to anchor the pins 116 in the bone.The surgical instrument may include an internal hammering device to lockthe anchoring pins 116 to the bone when the alignment is correct, or thesurgical instrument can include a portion, such as an anvil 118, thatcan be hammered to drive the pins 116 into the bone.

Once the anchoring device 115 is driven onto the bone, the anchoringdevice constrains the movement of the surgical instrument. Specifically,the anchoring device 115 limits lateral movement of the surgicalinstrument relative to the bone. However, the anchoring device allowsthe rotational of the surgical instrument relative to the bone, andpreferably, at least some axial displacement of the surgical instrumenttoward the bone. In this way, the anchoring device allows the cuttingblade to be aligned and maintained with the proper location to begin acut. At the same time, the anchoring device 115 allows the surgicalinstrument to rotate so that the cutting blade can be aligned with theplane of the proper cut and advanced into the bone.

As described below, during a cut, the anchor 115 may be configured tocollapse. Accordingly, to anchor the pins into the bone, the anchor 115includes a brake or a lock to lock the anchor in an extended positionwhile the pins are anchored into the bone.

Once the anchor 115 is anchored to the bone, the surgeon starts the tooland the cutting blade 102 is driven into the bone. The lock or brake onthe anchor is released to allow the anchor to collapse during a cut.Specifically, the anchor 115 is configured so that it can collapse ortelescope as the saw is moved forward during the procedure. In otherwords, the pins 116 remain in engagement with the tissue (e.g. bone) andthe anchor 115 collapses as the saw move forward relative to the pins.In this way, the pins 116 anchor the cutting blade 102 as the cuttingblade progresses through a cut.

As described above, the anchoring device includes a flat bar andretractable pins. However, the configuration of the anchor can varybased on a number of criteria, including, but not limited to design,friction and heat requirements, sterilization needs etc. For instance,rather than being an elongated flat bar, the anchor may comprise a pairof elongated cylindrical rods spaced apart from one another. The ends ofthe rods may be pointed to facilitate anchoring the anchor into thebone, as shown in FIG. 5 .

Additionally, the anchoring device need not include retractableelements. Instead, the anchoring device may be a rigid element that isremovable from the surgical instrument. In use, the anchoring device isdriven into the bone to anchor the surgical instrument relative to thebone. The surgical instrument is then operated to start a cut. After thecut is started, the surgical tool is withdrawn and the anchoring elementis removed from the surgical tool. The saw kerf that was started is thenused to guide the rest of the cut.

Tool Bending & Deflection Correction

As described above, the tracking system 120 can be used to detect andmonitor the position of either a surgical tool 100 or a surgical robot200. One issue in correctly navigating the surgical tool or the robot isthe need for an accurate assessment of the position and orientation ofthe surgical tool or robot. Specifically, although a number of markers105 may be used to identify the position of a tool, markers aretypically not applied to the tip of a tool, particularly if the tool isa cutting tool. Instead, the position of the tool is determined and theposition of the cutting tip is calculated based on the known geometry ofthe tool, and the presumption that the tool is a rigid element. However,during use, the tool may deflect or deform so that the actual positionof the cutting tip may not correspond to the presumed position of thecutting tip. Therefore, the correlation between the actual tissue beingcut and the virtual model do not match. In other words, based on thedata received from the position detection device the OR computer 80 maydetermine that a certain portion of tissue is resected, however, due totool deflection the actual tissue resected may be different.

The system may compensate for the tool bending or deflection in one ofseveral ways. Using system compensation, the system can monitor andcalculate the tool bending, and then manipulate the tracking data tocompensate for the calculated bending. Alternatively, using on-boardcompensation, the compensation calculations are determined and affectedon-board the surgical instrument.

Using system compensation, the tool may include a sensor for detectingdeflection or deformation of the tool. For instance, referring to FIG. 2, a surgical tool 100 is illustrated, having a cutting blade 102. Thesurgical tool 100 reciprocates the cutting blade during operation. Asensor in the form of a load-cell 104 included in the saw detects theforce and/or torque applied to the blade. Alternatively, a piezoelectricsensor may be connected directly to the blade to detect the force and/ortorque applied to the blade. The measured force or torque is used topredict the distance “d” that the blade bends. Specifically, propertiesof the cutting blade 102 are stored. Based on the predefined cuttingtool properties and the measured force or torque, the amount of bendingis calculated. The calculated amount of bending approximates thedistance “d” and is used as a compensation factor to adjust the positionand orientation of the cutting tool detected by the position detectiondevice 120. Specifically, not only will the system process the trackingdata to compensate for the position of the cutting tool, the system willalso process the tracking data to compensate for the angle of thedeflected cutting tool.

The advantage of using system compensation is that the alterations tothe surgical instrument are minimized. However, system compensationrequires that the system be programmed to alter the processing of thetracking data, as discussed above. While this may not be a barrier for anew system, users that already have a system with a tracking system maybe hesitant to adopt a process that requires alteration of the trackingsoftware or the software that processes the tracking data. Accordingly,it may be desirable to make the bending compensation adjustment on boardthe surgical instrument. The modified surgical instrument can then beused with the user's existing tracking system without any furthermodifications.

In the on-board approach, the surgical instrument includes an onboardprocessor that calculates the tool deflection. Based on the calculateddeflection, the surgical instrument manipulates the tracking element(s)to compensate for the deflection. In this way, the position detectiondevice 120 will detect the compensated position of the cutting tool,which will reflect the actual position and orientation of the deflectedcutting tool.

Referring again to FIG. 2 , the surgical tool 100 may include aprocessor 106 operable to receive signals from the load cell 104indicative of the force applied to the cutting blade. Based on the datareceived from the load cell, the processor 106 calculates the deflection“d” of the tip of the cutting tool 102 and the angle at which the bladeis deflected. The system, then manipulates the tracking element on thesurgical instrument.

In the present instance, the surgical tool includes a reference frameonto which a plurality of markers 105 are mounted. As describedpreviously, the tracking system 120 detects the position of the markersto determine the location and orientation of the surgical tool.

In a system compensation design, the frame is typically rigidly mountedto the surgical tool so that the position of the markers relative to therest of the tool is fixed. However, as shown in FIG. 2 , the frame 107may be movably connected to the surgical tool 100. Although the freedomof movement of the frame may be limited, preferably the frame isconnected to the surgical frame by a connection that provides at leasttwo degrees of freedom, such as a universal joint. Furthermore, in thepresent instance, the frame is extendable and retractable to alter thelength of the frame to properly compensate for the position andorientation of the deflected blade.

Connected to the frame 107 are a plurality of actuators or deflectors108 that control the position of the frame. The actuators 108 are inelectrical communication with the processor 106, and preferably theprocessor 106 independently controls the operation of each actuator.

The processor 106 controls the operation of the various deflectors 108based on the signals received from the sensor 104. Specifically, asdescribed above, the processor 106 calculates the deflection “d” of thetip of the cutting tool based on the signal received from the sensor104. Based on the calculated deflection, the processor determines theappropriate compensation to the position of the frame to compensate forthe deflection of the cutting tool 102. The processor then controls theoperation of the actuators 108 to re-position the frame. For instance,in the example illustrated in FIG. 2 , the cutting tool is deflected anamount “d” in a clockwise direction. Accordingly, the actuators 108reposition the frame 107 to displace the markers 105 an amount “d” in aclockwise direction. Additionally, the vertical actuator is operated toalter the height of the frame 107 to compensate for the proper plane ofthe deflected tool. The position detection device 120 then detects theposition of the surgical tool at the compensated position so that nofurther calculations are necessary to monitor the position of thedeflected cutting tool.

By utilizing an on board deflection compensation, the system canincorporate deflection compensation, while still allowing the surgicaltool to be used with a variety of commercially available positiondetection devices without the need to modify the software used by suchdevices.

Although the foregoing example describes the onboard compensationfeature as utilizing a plurality of actuators to reposition a referenceframe, the configuration of the compensation elements may vary dependingon the configuration of the position detection elements used.

For instance, other position detection devices may be used in thesystem, such as systems that include electromagnetic sensors, ultrasoundelements or accelerometers. When such elements are utilized, thecompensation features may either vary the position of the element or itmay vary the data provided by such elements in response to the datareceived regarding the load on the cutting tool.

Intelligent Control of Surgical Instrument

As described previously, the present system 50 can be used to performguided freehand surgery in which a model of the patient is provided,along with a model of the surgical tool and the models can be used toguide the surgeon during the actual procedure. For instance, the patientmodel may include a portion identified as tissue to be resected. Thesystem tracks the movement of the surgical tool 100, so that when thesurgeon moves the tool, the system displays the movement of the tool inreal time on the monitor. In this way, the surgeon can align the toolwith the patient by aligning the model of the tool with the portion ofthe patient model identified for resection. In this way, the surgeon canfollow the onscreen guidance to resect a portion of tissue.

During the procedure, the system may control or modulate the surgicalinstrument in response to the position of the surgical instrument.Specifically, as discussed previously, the system may track the positionand orientation of the surgical instrument relative to the patient. Ifthe surgical instrument is not in the proper position or orientationrelative to the patient, the system may control the surgical instrument,such as by stopping the instrument to ensure that the surgicalinstrument does not operate on the wrong portion of tissue.

Further still, in an alternate design, the system can intelligentlycontrol the operation of the surgical instrument based on additionaldata, such as the degree of mis-alignment or the location of theincorrectly positioned instrument.

For example, if the surgical instrument is in a position that does notcorrespond to the desired cut, it may not be a location that warrantsautomatically shutting off the instrument. For instance, the surgeon maybe holding the surgical instrument so that the saw blade is away fromthe patient and in mid-air. In such a position, the instrument is not inthe correct position to make a cut, but the instrument will not to anyharm in the air. Therefore, there is no need for the system to controlthe operation of the instrument. Conversely, if the instrument ispositioned adjacent the patient's tissue and it is improperly locatedand/or oriented. It may be desirable to control the operation of theinstrument to prevent the surgeon from cutting tissue erroneously.Accordingly, the system may control the operation of the surgicalinstrument in response to the location and orientation of the surgicalinstrument relative to the patient, combined with data about the areaaround the surgical instrument and whether the area around the surgicalinstrument can be damaged.

In addition, in some instances, the surgical instrument may not beproperly oriented or positioned, however themisalignment/misorienatation may be minor. A minor error may not warrantthe degree of automated override that a significant misalignmentwarrants. Accordingly, the operation of the surgical instrument may becontrolled in response to the degree of deviation from the desiredposition or orientation of the surgical instrument. Specifically, thedegree of control may correlate to the degree of error. The greater theerror, the greater the control. For example, a minor error in alignmentmay cause the system to attenuate the rate of the saw by a minor amount,whereas a substantial misalignment of the saw in a critical area oftissue may cause the system to stop the saw.

When controlling the operation of the surgical instrument, the controlcan either affect the range of modulation or the control can actuallymodulate the surgical instrument. For example, if the surgicalinstrument is improperly positioned, the saw may be controlled so thatit can only be operated by 0-50% normal speed. In other words, theinstrument can be operated in an attenuated range. Alternatively, theinstrument could be controlled so that the instrument operates at anattenuated speed, such as 50%.

The control or attenuation of the surgical instrument may be based oncontrol signals received from the OR computer based on pre-defined datafor the procedure and data from the tracking system. Alternatively, thesurgical instrument may include an onboard processor that determines theproper control based on the tracking data and pre-defined data for theprocedure.

Optional Features for the Surgical Instrument

As discussed previously, the surgical instrument 100 includes a cuttingtool 102 and a tracking element, such as a frame 107 with passivemarkers 105, as shown in FIGS. 1 & 2 . However, the surgical instrumentmay include a variety of optional features.

For instance, referring to FIG. 3 an alternate embodiment of surgicalinstrument 500 is illustrated along with corresponding elements for asystem. The surgical instrument 500 is operable to assist in automatedsurgery in a surgical suite as discussed above in connection with thesurgical instrument 100 described above. For instance, as describedabove, the system may include a tracking system 120 that operates todetect the position of the surgical instrument 500 relative to thepatient. In the present instance, the position detection device detectsthe position of one or more markers 505 on the surgical instrument andone or more markers connected to the patient. Although the instrumentillustrates the markers on a frame as with the first embodiment, themarkers need not be mounted on a frame. Instead, as shown in FIG. 20 ,the markers 605 may be embedded in the structure of the surgicalinstrument 600, such as on the housing 610 away from the gripping area602, or on the barrel or top portion of the surgical instrument. Ifembedded markers are utilized, the markers 605 include a portion that isreadily visible to either transmit a signal or light if they are activemarkers, or reflect light or other signal if the markers are passivemarkers. In the present instance, the markers 605 are passive markersthat are embedded in the surgical instrument. The markers are located onmultiple positions of the instrument and on multiple faces, so that thetracking system can identify the orientation of the instrumentregardless of which face of the instrument is facing the trackingsystem.

In addition to other aspects, the surgical instrument 500 incorporates anumber of features on board the instrument itself so that the instrumentcan be used to perform a number of functions independent of theprocessing done by the OR computer 80. Additionally, the surgicalinstrument may incorporate wireless communication with the OR computer80.

Referring to FIG. 3 the surgical instrument 500 includes a tool, such asa saw 510, a microcontroller 515 for monitoring and controllingoperation of the tool 510, and a wireless unit 520. The instrument 500also includes an antenna 525. The wireless unit 520 and antenna 525allow the instrument to send data to the OR computer 80 regardingmultiple status parameters, such as blade bending, saw speed and batterycharge. In addition, the OR computer 80 includes a wireless unit 86,such as a bluetooth wireless element, and an antenna 87. The wirelessunit 86 and antenna 87 allow the OR computer to send and receive datawirelessly to and from the surgical instrument 500.

As described previously, the OR computer 80 may be used to guide thesurgeon's operation of the surgical tool during a procedure. Forinstance, the system may track the position of the surgical tool in realtime and turn on or off the surgical tool depending on whether the toolis in proper alignment. For instance, if the system detects that thesurgical tool is adjacent an area to be resected, the system may send asignal wirelessly to the tool. If the tool does not receive such asignal, the tool will not operate. Specifically, the surgical tool mayhave a manual switch that the surgeon can manually turn on to operatethe tool. However, the tool will only run if both the manual switch isswitched to the on position and if the tool also receives a signalindicating that the tool is properly positioned to perform a procedure.If either the surgeon switches the tool off or if the tool does notreceive a signal indicating that the tool is properly positioned, thetool will not turn on for cutting.

As described above, the tool 500 may receive signals wirelessly tocontrol operation of the tool. In addition to signals controlling theon/off function of the tool, signals may also be used to control otheroperation of the tool. For instance, the tool may receive signals thatoperate to control the speed of the tool. For example, as describedabove, the system may track the position of the tool, so that the systemcan track whether the tool is adjacent a cutting boundary for a desiredprocedure. As the tool approaches the boundary, the system may send asignal to the tool indicating that the tool should be attenuated toreduce the speed of the tool. The circuitry in the tool 500 thenattenuates the operation of the tool in response to the wireless signal.

In addition to the system controlling the surgical instrument viawireless signals, the surgical instrument may control operation of thesystem via wireless signals For instance, the surgical tool may includevarious actuators, such as buttons, a joystick or a mouse ball. Theoperation of such actuators may be used as input signals to controloperation of the OR computer. For example, operation of a joystick onthe surgical tool 500 may send signals to the OR computer 80, causingthe graphics displayed on the display 85 to scroll in a particulardirection. Similarly, one or more buttons can be programmed to sendwireless signals to change the perspective or magnification of thegraphic being displayed.

In addition to including actuators, the surgical tool 500 may include adisplay 530 or view screen as shown in FIG. 16 . Specifically, asdescribed above, the tool may include a wireless connection forreceiving data from the OR computer 80. The OR computer may transmitgraphics data to the tool so that the display 530 may display the samegraphics as are displayed on the main OR computer 80 display 85.Alternatively, the display 530 may display an alternate view to thegraphic being displayed on the OR computer display 85. For instance, thesmall screen may show just a portion of the image shown on the largedisplay 85. The small area may be automatically determined based on thearea of interest in the view on the main display. Alternatively, thesystem may incorporate a number of pre-defined or user defined views,similar to the pre-defined views discussed above. The pre-defined viewsmay be an entire list of views that are defined for the small screen.Additionally, as with the main display, the surgical instrument may beconfigured to automatically change the view based on the position andorientation of the surgical instrument, or in response to the view beingshown on the main display 85. Further still, as shown in FIG. 20 , theonboard screen 630 may be positioned so that it is in-line with thecutting instrument, and the small display may include one or morealignment elements on the view. For instance, the view on the onboarddisplay that includes alignment lines or indicators that show angularalignment, such as roll, pitch etc. Still further, the onboard displaymay include lines such as a line showing where the surgical instrumentshould be located along with a line showing where the surgicalinstrument is actually located. Further still, the onboard screen may bea touch screen to allow input controls directly through the onboardscreen. In this way, the display screen 530 may be used to guide thesurgeon during a procedure in the same way that the OR computer display85 may be used to guide the surgeon.

As previously discussed, preferably a pointer is provided foridentifying reference points on the patient. Although the pointer hasbeen described as a separate element, the pointer may be integrated intothe surgical tool. For instance, since the configuration of the sawblade is known, the tip of the saw blade can operate as a pointer.Alternatively, a dedicated pointer may be incorporated onto the surgicaltool. It may be desirable to configure the pointer so that it can beextended and retracted as necessary so that the pointer can be readilyused, while not interfering with the operation of the cutting toolduring a procedure.

The operation of the pointer element may operate in conjunction with anactuator on the surgical tool. For instance, the tool may include abutton for indicating that the pointer is positioned at a referencepoint. When the surgeon positions the pointing element at a point to beregistered, the surgeon simultaneously presses the button, sending asignal to the OR computer indicating that the point is to be registeredas a reference point. The OR computer detects the position of thesurgical tool as determined by the position detection device, and storesthe data regarding the location of the reference point. In this way, theOR computer stores information regarding the position of the surgicaltool in response to actuation of the button on the surgical tool.

It will be recognized by those skilled in the art that changes ormodifications may be made to the above-described embodiments withoutdeparting from the broad inventive concepts of the invention. It shouldtherefore be understood that this invention is not limited to theparticular embodiments described herein, but is intended to include allchanges and modifications that are within the scope and spirit of theinvention as set forth in the claims.

What is claimed is:
 1. A method for guiding a freehand surgicalprocedure using a freehand surgical saw, the method comprising steps of:creating an electronic three dimensional representation of a portion ofa patient to which the guided freehand surgical procedure is to beperformed; identifying a portion of the electronic three dimensionalrepresentation of the portion of the patient corresponding to a portionof tissue for which the guided freehand surgical procedure is to beperformed to form a planar cut in the portion of the patient; displayinga three dimensional surgical scene that depicts an orientation of theelectronic three dimensional representation of the portion of thepatient and further depicts an electronic three dimensionalrepresentation of a blade of the freehand surgical saw during the guidedfreehand surgical procedure; altering an electronic representation of aboundary of the blade of the freehand surgical saw by elongating theelectronic three dimensional representation of the blade along a cuttingplane within the three dimensional surgical scene to aid in alignmentwith the orientation of the electronic three dimensional representationof the portion of the patient; tracking the freehand surgical saw whileperforming the guided freehand surgical procedure to obtain dataregarding the position and orientation of the freehand surgical sawblade relative to the portion of the patient; varying the display of theelectronic three dimensional representation of the portion of thepatient and the electronic three dimensional representation of the bladein response to the position and orientation data obtained during thetracking step; and assessing an aspect of the planar cut.
 2. The methodof claim 1, wherein the step of varying the display comprises varyingthe orientation of the electronic three dimensional representation ofthe portion of the patient and the electronic three dimensionalrepresentation of the blade displayed from an automatic selection basedon the location of the freehand surgical saw relative to the portion ofthe patient.
 3. The method of claim 2, wherein the automatic selectiondisplays a sagittal view of the portion of the patient, a lateral viewof the portion of the patient, or a medial view of the portion of thepatient.
 4. The method of claim 1, further comprising a step of:tracking data for a user regarding information input into a surgicalsystem by the user in connection with one or more surgical procedures,wherein the step of varying the display comprises the step of varyingthe display in response to both the data obtained during the step oftracking the tool and the data obtained during the step of tracking thedata for the user.
 5. The method of claim 1, further comprising a stepof: manually altering the orientation of the electronic threedimensional representation of the portion of the patient and theelectronic three dimensional representation of the blade in the displayon-board the freehand surgical saw during the guided freehand surgicalprocedure.
 6. The method of claim 5, wherein the step of manuallyaltering the orientation further comprises operating a control on thefreehand surgical saw.
 7. The method of claim 1, further comprising astep of: modulating the operation of the freehand surgical saw duringthe step of performing the guided freehand surgical procedure.
 8. Themethod of claim 7, wherein the step of modulating the operation of thefreehand surgical saw inhibits the operation of the freehand surgicalsaw when the saw blade is in an improper position or an improperorientation.
 9. The method of claim 7, wherein the step of modulatingthe operation of the freehand surgical saw permits the operation of thefreehand surgical saw when the saw blade is in a proper position or aproper orientation.
 10. The method of claim 1, wherein during theperforming the guided freehand surgical procedure step, the displayingstep adjusts to represent the electronic three dimensionalrepresentation of the blade of the freehand surgical saw moving relativeto the progress of forming the planar cut in the portion of the patient.11. The method of claim 1, wherein the displaying step further comprisesaltering a shape used to represent the orientation of the electronicthree dimensional representation of the blade of the freehand surgicalsaw during the guided freehand surgical procedure, wherein the shape ofthe blade is altered to indicate a degree of twisting of the bladerelative to the cutting plane.
 12. The method of claim 11, wherein theshape used to represent the orientation of the electronic threedimensional representation of the blade of the freehand surgical saw isa line when the blade of the freehand surgical saw is aligned.
 13. Themethod of claim 11, wherein the shape used to represent the orientationof the electronic three dimensional representation of the blade of thefreehand surgical saw is rounded when the blade of the freehand surgicalsaw is misaligned, wherein the roundedness of the blade is altered basedon a variation of an angle between the blade and the cutting plane. 14.The method of claim 1, wherein after the altering step the electronicrepresentation of a boundary of the blade of the freehand surgical sawincludes an opaque portion and a semi-transparent portion.
 15. Themethod of claim 14, wherein the opaque portion corresponds to actualsurgical saw and the semi-transparent portion corresponds to the aspectof the blade to aid in alignment.
 16. The method of claim 1, wherein theassessing the aspect of the planar cut includes: evaluating whether asurface finish of the planar cut will bond with a prosthesis; evaluatinga fit of the planar cut to the prosthesis; evaluating an alignment ofthe planar cut with the prosthesis; and evaluating an accuracy of theplanar cut for engagement with the prosthesis.
 17. A system for guidinga freehand surgical procedure using a freehand surgical saw, the systemcomprising: means for creating an electronic three dimensionalrepresentation of a portion of a patient to which the guided freehandsurgical procedure is to be performed; means for identifying a portionof the electronic three dimensional representation of the portion of thepatient corresponding to a portion of tissue for which the guidedfreehand surgical procedure is to be performed to form a planar cut inthe portion of the patient; means for displaying a three dimensionalsurgical scene that depicts an orientation of the electronic threedimensional representation of the portion of the patient and furtherdepicts an electronic three dimensional representation of a blade of thefreehand surgical saw during the guided freehand surgical procedure;means for altering an electronic representation of a boundary of theblade of the freehand surgical saw by elongating the electronic threedimensional representation of the blade along a cutting plane within thethree dimensional surgical scene to aid in alignment with theorientation of the electronic three dimensional representation of theportion of the patient; means for tracking the freehand surgical sawwhile performing the guided freehand surgical procedure to obtain dataregarding the position and orientation of the freehand surgical sawblade relative to the portion of the patient; means for varying thedisplay of the electronic three dimensional representation of theportion of the patient and the electronic three dimensionalrepresentation of the blade in response to the position and orientationdata; and means for assessing an aspect of the planar cut.
 18. Thesystem of claim 17, further comprising: means for altering a shape usedto represent the orientation of the electronic three dimensionalrepresentation of the blade of the freehand surgical saw during theguided freehand surgical procedure, wherein the shape of the blade isaltered to indicate a degree of twisting of the blade relative to thecutting plane.
 19. The system of claim 18, wherein the shape used torepresent the orientation of the electronic three dimensionalrepresentation of the blade of the freehand surgical saw is a line whenthe blade of the freehand surgical saw is aligned.
 20. The system ofclaim 18, wherein the shape used to represent the orientation of theelectronic three dimensional representation of the blade of the freehandsurgical saw is rounded when the blade of the freehand surgical saw ismisaligned, wherein the roundedness of the blade is altered based on avariation of an angle between the blade and the cutting plane.